Switch css and js bundling to Dart SASS and esbuild

[rahearn]

🎫 Addresses issue: #96

closes #96

🛠 Summary of changes

• swap out webpack in favor of esbuild
• swap out postcss in favor of dart-sass
• switch around i18n-js generator to play nicer with the default esbuild path globbing

both swaps are more performant and require less configuration than their predecessors. In addition, this lets us drop a weird dependency on one of my forks of a long-abandoned project

the template still supports passing --javascript=webpack (since it wasn't really doing anything for that in the first place), but --css=postcss won't work out of the box anymore.

📜 Testing Plan

How would a peer test this work?

• create a new application
• verify that bin/rails assets:precompile works properly
• run bin/rails assets:clobber to get rid of static assets
• run bin/dev to start the local server and verify site looks proper in browser
• make changes to application.css and/or application.js and verify that changes are properly reflected on a reload

[pauldoomgov]

LGTM!

[zjrgov]

This is delightfully simple for a (multi) build tool transition.

[zjrgov]

:-)