Update JS build dependencies and modernize theme JS

[kadamwhite]

wp-scripts is getting updates targeted at Node v20, and sass is included internally within the @wordpress/scripts package dependencies. This PR updates the Interconnection build to use Node 20 and the latest version of wp-scripts, and assumes the sass executable will be provided internally by the wp dependency.

[!NOTE] We can't rely on wp-scripts build because that configuration makes too many assumptions about the structure of our project, and targets block-based builds. It isn't a good fit for a CSS-only theme build, especially with RTLCSS in play.

Then, to address #4 this fixes existing issues with npm run lint:js and then

• wpdates the eslint config to the latest wp-scripts version
• removes unneeded JS code
• updates bundled headroom script
• updates JS for modern browsers (updating any upstream code from latest released _s theme)
• implements a GitHub Action to run the JS build and lint JS files on PR, copied from the Shiro theme
• pins GH Action dependencies to avoid hypothetical supply chain attacks

[kadamwhite]

Merged to develop to test build, and it completed correctly: https://github.com/wikimedia/interconnection-wordpress-theme/actions/runs/8990525292/job/24696037305

[kadamwhite]

Inline annotations are working for JS!:

Reverting 59b6170

[kadamwhite]

@goldenapples Oh fun, look at this: https://developer.mozilla.org/en-US/docs/Web/API/Window/event Window.event will be the same as an event function argument, but was deprecated, so the change you flagged here was functionally more correct but will not have changed actual behavior. Explains a lot.