Add/GitHub actions

[jeffpaul]

Description of the Change

Adds our GitHub Actions, mainly so we can continue to easily bump the WP tested-up-to value with future WordPress major releases. Also includes additional changes related to likely having to ship a minor release to get these changes to WP.org.

Alternate Designs

keep as-is and handle tested-up-to updates via SVN (no thanks).

Benefits

Ensures we can more easily update the plugin and asset/readme changes in the future.

Possible Drawbacks

A plugin that only targets the classic editor will continue to have less and less relevance as focus shifts, rightly, to the block editor.

Verification Process

Manually verified via VS Code and GitHub Desktop UI.

Checklist:

• [x] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests passed.

Applicable Issues

n/a

Changelog Entry

• Docs docs docs

[jeffpaul]

@dinhtungdu this should be good to merge now, yeah?

[dinhtungdu]

@jeffpaul I'm sorry for the late reply. Yes, this is ready to be merged. I opened #16 to fix the lining issues.