Plugin Directory: Remove unused React code

[ryelle]

As part of the Plugin Directory theme refresh #1719, the theme UI was rewritten into a React app. It was disabled, and has been unused waiting to be re-enabled. This would need some investment to get it to a launchable state (best practices have changed, packages would need updating, etc). Since development time is better spent on other features, this code should be removed.

Remove the react theme code, leaving only the screenshot component (the only part of the app currently in use). This leaves most of the CSS in place, but collapses it down into fewer folders. This also updates the build process to use wp-scripts for the JS, so we can get the dependency extraction code.

npm run build or npm run watch can be used to build the CSS and JS.

https://meta.trac.wordpress.org/ticket/5431

[ryelle]

In 10605:

Plugin Directory: Remove unused React code.

As part of the Plugin Directory refresh #1719, the UI was rewritten into a React app, but currently only the screenshot slider uses React. This removes all the React JS except for the screenshots, and flattens the Sass into less complex files. Additionally it updates the packages and JS build process to use @wordpress/scripts.

Fixes #5431.

Committed in r10605.