CI Pipeline: Refactor CI pipeline files

[brbrr]

This is a follow up to GH Actions migration #17902

Changes proposed in this Pull Request:

• move CI related files into .github folder
• Refactor these files into a smaller, single purpose scripts

Jetpack product discussion

n/a

Does this pull request change what data or activity we track or use?

n/a

Testing instructions:

• Does it still works?
• Code looks good?

Proposed changelog entry for your changes:

• n/a

[jetpackbot]

Scheduled Jetpack release: December 10, 2020.
Scheduled code freeze: December 3, 2020

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Generated by 🚫 dangerJS against 4b017d6

[anomiex]

While the test jobs are running, they're not actually running the tests. See inline comments for details.

I've suggested some pretty significant changes inline. Feel free to disagree, but IMO it'd be better to do more of the work in the workflow yaml files instead of in scripts to take advantage of the Github UI's grouping of steps. That'd get rid of the code-coverage.sh, phpunit.sh, and includes.sh files.

[brbrr]

@anomiex Thanks for the thorough review! (I think) I addressed all your comments and move away from shell scripts completely.