Add and resolve errors from linting actions

[jeffpaul]

We'll want to add PHPCS and ESLint actions and work to resolve errors identified from them:

• https://github.com/10up/insert-special-characters/blob/develop/.github/workflows/phpcs.yml or perhaps using https://github.com/10up/wpcs-action
• https://github.com/10up/insert-special-characters/blob/develop/.github/workflows/eslint.yml
• Resolve all PHPCS issues
• Resolve all Eslint issues

[dkotter]

I can help get these Actions in place but as far as resolving issues, I'd suggest tackling this in two different phases:

1. Open a PR to fix all warnings, which are typically easier things to fix, like spacing
2. Open a PR to fix all errors, which generally need a little more effort/review

^ These apply more to PHPCS than eslint, though we can probably handle both in similar fashion.

I see they're bundling some jQuery scripts so we'll want to exclude those from eslint.

[dkotter]

And I guess thinking about it more, any assets (like JS or CSS files) that are things bundled in WordPress Core (for instance, seeing a bunch of jQuery UI things) we should look to see if we can remove those from our codebase and just rely on loading those from Core