Improve keyboard accessibility of the editable Ace editors

Problem/Motivation

When the Ace editor replaces a textarea on an edit form (the text-editor integration and the field widget), Ace binds the Tab key to indent and Shift+Tab to outdent. A keyboard-only user who tabs into the editor cannot tab back out to reach the text-format select, the Save button or the next field. This is a WCAG 2.1.2 (No Keyboard Trap, Level A) problem. Read-only editors (the formatter and filter) are not affected.

Steps to reproduce

1. Edit a node whose body uses the Ace text editor, using only the keyboard.
2. Tab into the editor. Tab no longer moves focus out of it.

Proposed resolution

• Enable Ace's own keyboard-accessibility mode on the editable editors: after creating the editor call editor.setOption('enableKeyboardAccessibility', true). This frees Tab and adds Esc to leave the editor. Guarded so older Ace builds that do not know the option are unaffected.
• Follow-up (this issue or a child): give the replacement editor an accessible name and keep the field label associated with it.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ UX/UI designer responsibilities
• ❌ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• Tab and Shift+Tab move focus out of an editable Ace editor; Esc leaves it.

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Editable Ace editors no longer trap the keyboard: Tab moves out of the editor and Esc leaves it.