Add automated linting for PHP and JS (CI)

[helen]

Is your enhancement related to a problem? Please describe.
While there is a precommit hook, it currently only runs phpcs (possibly needs to be updated) and it's still possible for somebody to ignore that and push to a branch and open a PR with those errors.

Describe the solution you'd like
I would like to see PHPCS and ESLint running in GitHub Actions with annotations, see https://github.com/helen/wordpress-develop/pull/1/files for an example.

Designs
n/a

Describe alternatives you've considered
n/a

Additional context
See #209

[dinhtungdu]

Closed by #226