upkeep: replace grunt with webpack

[Sidsector9]

Closes #203

Description of the Change

This PR does the following:

1. Replace grunt with wp-scripts
2. Bump node to 16
3. Bump composer to v2
4. Bump wp-env to 5.4.0
5. Refactor E2E tests: a. Reduces pipeline time. b. Achieves test isolation. c. Reduces time spent on UI navigation where it is unnecessary.
6. Fix PHPCS linting errors

Verification Process

• Manual testing.
• PHPUnit cleared.
• E2E testing cleared.

Checklist:

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

Changelog

Security: Bump wp-env to 5.4.0. Change: Refactored E2E tests to reduce run-time. Change: Migrate from Grunt to wp-scripts.

[Sidsector9]

@jeffpaul as detailed in the PR, can we proceed with the moderate vulnerability for now?

[jeffpaul]

@Sidsector9 seems like that's our best path forward at this point