refactor(assets): swap webpack for esbuild

- Remove all webpack and babel related npm dependencies
- Add esbuild and esbuild-plugin-postcss2 as a dependency
- Add a custom `build.js` script to build css and js with esbuild
  and copy static files to the priv/static directory
- Modify `config/dev.exs` to use that build script as a watcher instead
  of webpack
- Modify `package.json` to use that build script in instead of webpack
- Modify `KetbinWeb.Endpoint` to serve the assets directory instead of
  css and js directories from static
- Modify the `app.html.eex` layout to use `assets` directory instead of
  separate css and js directories

Signed-off-by: Akshit Garg <garg.akshit@gmail.com>

config/dev.exs

@@ -13,10 +13,7 @@ config :ketbin, KetbinWeb.Endpoint,
   check_origin: false,
   watchers: [
     node: [
-      "node_modules/webpack/bin/webpack.js",
-      "--mode",
-      "development",
-      "--watch-stdin",
+      "build.js",
       cd: Path.expand("../assets", __DIR__)
     ]
   ]