Split app.js logic and do bundling with esbuild

[reynaldichernando]

Fixes #9

• added esbuild for bundling
• split event handlers (js files) based on features

[reynaldichernando]

some notes

• we still have app.js right now because it has both logic for sidebar and router, currently its tightly coupled, so we might need to revisit this, will fix this with the router.js change
• we still manually add the jquery and highlight js via copy file, we have 2 options (open for discussion)
  • bundle them together into single JS (this we can just remove the copied file, and import from node modules, esbuild is pretty smart enough to include it in bundle)
  • do not bundle them, and just use cdn, like unpkg or jsdelivery
• once we do the jquery and highlight js, we no longer need to copy the entire assets/js since its bundled anyway, so we could remove the copy code

[reynaldichernando]

other than above comment, this change is net positive for both devex (separate files/concerns) and performance (15kb total bundle size, down from 24KB app.js)