Follow-up for #3205565: bring back visual annotation

Problem/Motivation

#3205565: Add language of parts plugin brought #1993928: Language of parts: Introduce a language toolbar button to CKEditor 5. Yay!

But #1993928: Language of parts: Introduce a language toolbar button had a crucial UX improvement on top of what CKEditor 5 provides, by adding some CSS.

See for yourself:

CKEditor 4

CKEditor 5

This was done through core/modules/ckeditor/css/plugins/language/ckeditor.language.css, whose contents are trivial:

/**
 * Show the user that a 'lang' tag has been applied by adding a thin dotted
 * border. We also append the value of the tag between brackets, for example:
 * '(en)'. Since the html element has a 'lang' attribute too we only target
 * elements within the html scope.
 */
html [lang] {
  outline: 1px dotted gray;
}
html [lang]:after {
  content: " ("attr(lang)")";
  color: #666;
  font-size: 10px;
}

Unfortunately, we cannot simply replicate this in CKEditor 5, because the CKEditor 5 architecture is completely different. We cannot load any CSS into a CKEditor 5 instance! (In that sense, #3194084: Support functionality equivalent to ckeditor_stylesheets is related too.)

Steps to reproduce

TBD

Proposed resolution

Before

After

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD