dev: use esbuild for app.js

[mastercactapus]

Description:
Replaces webpack with esbuild for building app.js.

• faster build
• simpler build & dev
• fixes localdev cache issues

Review:

• Check with HTTP_PREFIX set
• prod and non-prod mode (& /w cypress)
• changes should result in page refresh (may need hard refresh once/first if you were previously affected by cache bug)
• validate js files container GOALERT_VERSION banner

Out of Scope:
Cypress still uses a webpack plugin to build the test bundle and will be migrated separately.