Enforce phpcs linting rules before commit

### Summary
Enforce our phpcs guideline https://github.com/Automattic/woocommerce-services/blob/develop/phpcs.xml.dist on any new changes going forward.

### Output
- Update pre-commit git hook to run phpcs
- Add phpcs linting as a new action to GitHub Action
- Fix all phpcs errors in `classes/*`
- Fix all phpcs errors in `woocommerce-services.php`
- Check with Woo core to see what/which rules we should be using? Why isn't wordpress.security part of woocommerce-sniffs?

### Impact
It will require us to fix all phpcs warning/errors on the entire file as long as it is modified.

Start with phpcs.xml.dist, then inspect the existing pre-commit hook and GitHub Actions configuration to determine where linting belongs. Run PHPCS against classes/* and woocommerce-services.php, and consider the Woo core question about the applicable rules. Done means both checks run on changes and the listed files have no PHPCS errors.

Written by the indexing model from the issue text.