Feature/updated linting

[darylldoyle]

Description of the Change

This PR updates the linting within the repo to lint CSS/JS and PHP using the 10up standards.

• It adds in ESlint and StyleLint from the 10up toolkit.
• It adds lint-staged and husky to ensure that future PRs align with the code styles.
• It updates the linting Github Action to use the new configs
• It fixes a bunch of PHPCS errors that were being flagged.

How to test the Change

• Smoketest the plugin.
• Run linters
  • npm run lint-js
  • npm run lint-style
  • composer run lint

Changelog Entry

• Added - Additional linting.
• Changed - The linting standards to align with the 10up standards.
• Fixed - PHPCS errors/warnings.

Credits

Props @darylldoyle

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.