Fix adding/deleting table columns across sections

[talldan]

Description

Fixes #16046

Ensures adding table cells/columns results in cells being added across table sections (in the header, body and footer)..

How has this been tested?

• Added unit tests
• Added e2e tests

Manual Testing steps

1. Navigate to Posts > Add New
2. Insert a Table block
3. Choose Column/Row Count and press "Create Table" (specific values do not matter)
4. Toggle "Header section" in the Block Inspector "Table Settings" sidebar
5. Select a cell in the table
6. From the "Edit Table" Block Toolbar menu, select "Add Column Before" or "Add Column After"
7. Note that the last column does not include cells under its header

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[youknowriad]

Seems to work well for me. Thanks for the fix.

[ellatrix]

Ah, @youknowriad beat me to leaving a review. :)