chore: switch build system to esbuild

[ThatNerdSquared]

What Issue does this PR resolve? (Link to GitHub Issue, approved features and bugs will be given priority)

• completely removes create-react-app and the hidden dependencies/configs that come with it
• switches bundling/minification from webpack/babel to esbuild
  • speeds up build time by ~267x (from ~8-10s to ~30-60ms)
  • bundle size remains roughly the same
  • reduction in config size (86->30 lines)
  • dev builds now come unminified (for easier debugging)
  • prod builds now come without sourcemaps (for reduced bundle size) [this can be reverted if needed]
• new build commands
  • yarn build-chrome and yarn build-firefox prepare a fully optimized production build
  • yarn dev-chrome and yarn dev-firefox build the extension, then watch for updates to the files and rebuild the bundle when files are changed
• other small cleanups:
  • manifests and demo gif have been moved to an assets/ folder
  • extension is now built in public instead of build to reduce copying of icons
  • manifests across both browsers have been standardized
  • deleted unused images/build files
  • updated README build documentation

Please provide a video demo below, or a screenshot and description of the change.

There should be no visual changes, any change to actual extension visuals or functionality is considered a bug

Tag reviewers for the PR below.

@mlool