Mobile,Desktop: Fix #9200: Fix list renumbering and enable multiple selections

[personalizedrefrigerator]

Summary

This pull request does two things:

• Fixes a list renumbering issue when there are multiple selections (fixes Desktop: CM6 beta editor duplicates list items when indenting with multiple selections #9200)
• Enables multiple selections (fixes CodeMirror 6 beta editor: Multiple cursors are enabled, but missing the ability to alt+drag (as in the CodeMirror 5 editor) #9196) to allow testing the above manually,
  • On request, enabling multiple selections can be moved to a separate pull request.

Testing plan

This pull request has an automated test. However, it can be tested manually by:

1. Creating a numbered list:

1. this
2. is
3. a
4. test
5. of
6. lists

2. Adding cursors to lines 2, 3, and 4:
3. Pressing tab
4. Removing all cursors, then adding cursors to lines 3 and 4
5. Pressing tab
6. Adding a cursor to just line 4 (remove the cursor from line 3)
7. Pressing tab
8. Adding a cursor to line 5
9. Pressing shift-tab twice

After each step, each list should have the same numbering as it does in the viewer.

This has been successfully tested on Ubuntu 23.10.

[personalizedrefrigerator]

I'm unsure why this wasn't necessary to handle double de-indentation before.

I suspect previously sublists weren't renumbered with parent lists. However, selecting a parent list and pressing tab should have also caused sublists to be renumbered.