Improve table editing in content editor

What does this MR do and why?

Improve table editing in content editor

1. Migrate GlDropdown to GlDisclosureDropdown for the table cell context menu. #413044 (closed)
2. Allow selecting multiple rows and columns in a table to show relevant operations.
3. New operations:
   1. Split cell - available on a cell with rowspan > 1 or colspan > 1.
   2. Merge cells
4. Updated dropdown item labels: Now when you select multiple cells, the dropdown shows appropriate plural label:
   1. Merge X cells
   2. Delete X rows
   3. Delete X columns

Screenshots or screen recordings

Screen Recording 2023-05-23 at 18.31.39.mov

How to set up and validate locally

1. Create a wiki page
2. Click "Switch to rich text" to enable content editor.
3. Insert a new table from the table dropdown in the toolbar.
4. Use operations on a table cell as described in the video.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.