Desktop: Resolves #5762: Rich Text Editor: Add eight spaces when pressing tab

[personalizedrefrigerator]

Summary

This pull request adds the folllowing keyboard shortcuts to the Rich Text Editor:

1. Tab: Adds eight nonbreaking spaces before the cursor.
   • Adds before the cursor: This is similar to how LibreOffice Writer handles the tab key — a tab character is inserted just before the cursor.
   • If in a list or table, the previous navigation/indentation behavior is preserved.
2. Shift-Tab: Deletes up to eight spaces before the cursor.
   • If in a list or table, the previous navigation/indentation behavior is preserved.
3. Escape, then Tab: Moves focus to the first item out of the Rich Text Editor. (Accessibility).
4. Escape, then Shift-tab: Moves focus to the first item before the Rich Text Editor.

Why (nonbreaking) spaces?

1. Logic already exists to convert repeated nonbreaking spaces to Markdown.
2. Tab characters are treated as collapsible whitespace by default in HTML.

Resolves #5762.

Screen recording

Screencast.from.2024-08-15.19-40-55.webm

Testing

This pull request includes an automated Playwright test that verifies 1) Tab adds spaces in a paragraph, 2) shift-Tab removes spaces before the cursor, 3) Escape, then Tab moves the focus.

See the screen recording for manual testing information.