Fix compilation errors for TS6+

Problem/Motivation

Can't compile on latest node/npm/ts

<h3 id="summary-problem-motivation">Problem/Motivation</h3>
Can't compile on latest node/npm/ts

<code>
user007@user007-MacBook-Air ckeditor5_bootstrap_accordion % npm run build

> ckeditor5-bootstrap-accordion@1.0.1 build
> webpack

assets by status 28.6 KiB [cached] 1 asset
orphan modules 237 KiB [orphan] 413 modules
runtime modules 670 bytes 3 modules
built modules 61.6 KiB [built]
  cacheable modules 61.4 KiB
    modules by path ./ckeditor5_plugins/bootstrapAccordion/src/ 43.4 KiB 18 modules
    modules by path ./icons/*.svg 1.56 KiB 3 modules
    ./node_modules/@ckeditor/ckeditor5-html-support/src/utils.js + 12 modules 16.4 KiB [built] [code generated]
  modules by path delegated ./*.dll 210 bytes
    delegated ./core.js from dll-reference CKEditor5.dll 42 bytes [built] [code generated]
    delegated ./utils.js from dll-reference CKEditor5.dll 42 bytes [built] [code generated]
    delegated ./widget.js from dll-reference CKEditor5.dll 42 bytes [built] [code generated]
    delegated ./ui.js from dll-reference CKEditor5.dll 42 bytes [built] [code generated]
    delegated ./engine.js from dll-reference CKEditor5.dll 42 bytes [built] [code generated]
  external "CKEditor5.dll" 42 bytes [built] [code generated]

ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts
./ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts 19:18-23
[tsl] ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts(19,19)
      TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 @ ./ckeditor5_plugins/bootstrapAccordion/src/bootstrapaccordionediting.ts 12:65-125
 @ ./ckeditor5_plugins/bootstrapAccordion/src/bootstrapaccordion.ts 7:52-90
 @ ./ckeditor5_plugins/bootstrapAccordion/src/index.ts 6:45-76

ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts
./ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts 20:2-7
[tsl] ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts(20,3)
      TS2300: Duplicate identifier 'value'.
 @ ./ckeditor5_plugins/bootstrapAccordion/src/bootstrapaccordionediting.ts 12:65-125
 @ ./ckeditor5_plugins/bootstrapAccordion/src/bootstrapaccordion.ts 7:52-90
 @ ./ckeditor5_plugins/bootstrapAccordion/src/index.ts 6:45-76

ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts
./ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts 20:2-7
[tsl] ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts(20,3)
      TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 @ ./ckeditor5_plugins/bootstrapAccordion/src/bootstrapaccordionediting.ts 12:65-125
 @ ./ckeditor5_plugins/bootstrapAccordion/src/bootstrapaccordion.ts 7:52-90
 @ ./ckeditor5_plugins/bootstrapAccordion/src/index.ts 6:45-76

ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts
./ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts 20:2-7
[tsl] ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/commands/bootstrapaccordionfirstitemopencommand.ts(20,3)
      TS2717: Subsequent property declarations must have the same type.  Property 'value' must be of type 'boolean', but here has type 'false'.
 @ ./ckeditor5_plugins/bootstrapAccordion/src/bootstrapaccordionediting.ts 12:65-125
 @ ./ckeditor5_plugins/bootstrapAccordion/src/bootstrapaccordion.ts 7:52-90
 @ ./ckeditor5_plugins/bootstrapAccordion/src/index.ts 6:45-76

ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/integration/bootstrapaccordionclipboardpipeline.ts
./ckeditor5_plugins/bootstrapAccordion/src/integration/bootstrapaccordionclipboardpipeline.ts 32:50-62
[tsl] ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/integration/bootstrapaccordionclipboardpipeline.ts(32,51)
      TS2345: Argument of type 'import("/Users/user007/work/ckeditor5_bootstrap_accordion/node_modules/@ckeditor/ckeditor5-clipboard/node_modules/@ckeditor/ckeditor5-engine/src/view/documentfragment").ViewDocumentFragment' is not assignable to parameter of type 'import("/Users/user007/work/ckeditor5_bootstrap_accordion/node_modules/@ckeditor/ckeditor5-engine/src/view/documentfragment").ViewDocumentFragment'.
  The types of 'document.selection' are incompatible between these types.
    Type 'import("/Users/user007/work/ckeditor5_bootstrap_accordion/node_modules/@ckeditor/ckeditor5-clipboard/node_modules/@ckeditor/ckeditor5-engine/src/view/documentselection").ViewDocumentSelection' is not assignable to type 'import("/Users/user007/work/ckeditor5_bootstrap_accordion/node_modules/@ckeditor/ckeditor5-engine/src/view/documentselection").ViewDocumentSelection'.
      Types have separate declarations of a private property '_selection'.
 @ ./ckeditor5_plugins/bootstrapAccordion/src/index.ts 7:62-122

ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/integration/bootstrapaccordionclipboardpipeline.ts
./ckeditor5_plugins/bootstrapAccordion/src/integration/bootstrapaccordionclipboardpipeline.ts 38:6-18
[tsl] ERROR in /Users/user007/work/ckeditor5_bootstrap_accordion/ckeditor5_plugins/bootstrapAccordion/src/integration/bootstrapaccordionclipboardpipeline.ts(38,7)
      TS2322: Type 'import("/Users/user007/work/ckeditor5_bootstrap_accordion/node_modules/@ckeditor/ckeditor5-engine/src/view/documentfragment").ViewDocumentFragment' is not assignable to type 'import("/Users/user007/work/ckeditor5_bootstrap_accordion/node_modules/@ckeditor/ckeditor5-clipboard/node_modules/@ckeditor/ckeditor5-engine/src/view/documentfragment").ViewDocumentFragment'.
  The types of 'document.selection' are incompatible between these types.
    Type 'import("/Users/user007/work/ckeditor5_bootstrap_accordion/node_modules/@ckeditor/ckeditor5-engine/src/view/documentselection").ViewDocumentSelection' is not assignable to type 'import("/Users/user007/work/ckeditor5_bootstrap_accordion/node_modules/@ckeditor/ckeditor5-clipboard/node_modules/@ckeditor/ckeditor5-engine/src/view/documentselection").ViewDocumentSelection'.
      Types have separate declarations of a private property '_selection'.
 @ ./ckeditor5_plugins/bootstrapAccordion/src/index.ts 7:62-122

6 errors have detailed information that is not shown.
Use 'stats.errorDetails: true' resp. '--stats-error-details' to show it.

webpack 5.105.4 compiled with 6 errors in 1123 ms