Add composer and PHPCS for local linting

[cameronterry]

Description of the Change

Cleans up the NPM scripts and adds PHPCS through Composer, enabling linting for PHP in addition to CSS and JS; my assumption being this would be useful longer term than removing the Composer references.

I've removed the composer install --no-dev -o && portion from npm run build-release but kept the other references in npm run lint and npm run lint-release, as it has PHPCS will now be available.

Running npm run lint-php or composer run lint returns:

> composer run lint

> ./vendor/bin/phpcs . -p -s
.... 4 / 4 (100%)


Time: 175ms; Memory: 16MB

Closes #116

How to test the Change

1. Clone the repository.
2. Switch to an appropriate Node/NPM version (if required).
3. Run npm install.
4. Run npm run build-release.
5. And run npm run lint-php.
6. And then run npm run lint (this seemingly runs into an issue seen in one of the build actions, https://github.com/10up/insecure-content-warning/actions/runs/5336668720/jobs/9671677686, which I think is related to this, Update node and NPM versions #114?)

Changelog Entry

Added - Composer, with PHPCBF and PHPCS to aid with coding standards.
Changed/Fixed - Possibly error from running npm run build-release.

Credits

Props @cameronterry

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[peterwilsoncc]

Added a few notes inline, mainly to make sure we ignore a few more things.

[cameronterry]

Thank you for the CR, @peterwilsoncc, and my apologies for the late reply and tardiness of some of the code: I've added both your suggestions and resolved the conflict with the most recent changes in the develop branch.

[peterwilsoncc]

@cameronterry No need to apologise, we're all very busy. Thanks for the PR.

This LGMT

[peterwilsoncc]

@jeffpaul @dkotter I'm unable to merge as it looks like some required actions have been removed/renamed or stalled.

[jeffpaul]

@peterwilsoncc I think I've resolved those blocking required actions, but looks like composer files have conflicts to resolve before merging.

[peterwilsoncc]

@jeffpaul Conflicts have been resolved, tests are passing.