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>

assets/js/app.js

@@ -1,7 +1,7 @@
 // We need to import the CSS so that webpack will load it.
 // The MiniCssExtractPlugin is used to separate it out into
 // its own CSS file.
-import "../css/app.css"
+import "../css/app.css";
 
 // webpack automatically bundles all modules in your
 // entry points. Those entry points can be configured
@@ -12,4 +12,4 @@ import "../css/app.css"
 //     import {Socket} from "phoenix"
 //     import socket from "./socket"
 //
-import "phoenix_html"
+import "phoenix_html";