Prevent losing colspan on header when adding columns

[shsteimer]

Description

wrap addColumnBefore and addColumnAfter to ensure when doing these from block header, header maintains correct col span

Related Issue

Motivation and Context

just trying to make things easier to use and prevent modifications that don't make sense in context

How Has This Been Tested?

locally at http://localhost:3000/edit#/aem-sandbox/block-collection/drafts/shsteimer/blocks-

new unit tests on table modifications to document/confirm behaviors

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[auniverseaway]

Small organizational fix, otherwise this is good.

[auniverseaway]

I would follow the existing pattern and not write code directly in this file. The basics seem fine, though.

[shsteimer]

Refactored to use a plugin. Overall think it's a reasonably clean approach, but I won't be heart broken if you tell me this works as designed and isn't worth the trouble, as the default behavior does appear to match word