Set up JS linting for repo

[jjgrainger]

Set up JS linting via ESLint for this repository to highlight formatting issues as per WP coding standards.

[felixarntz]

@spacedmonkey Is that something you'd be interested in opening a PR for? We don't need any JS build process at this point, just setting up ESLint linting and formatting with Prettier for WP coding standards in JS (e.g. with @wordpress/scripts) would be the scope of this issue, and then preferably add it as a CI workflow too.

[spacedmonkey]

@felixarntz I put together a PR at #104. It is simple but works.