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>

lib/ketbin_web/templates/layout/app.html.eex

@@ -5,11 +5,11 @@
     <meta http-equiv="X-UA-Compatible" content="IE=edge"/>
     <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
     <title>Katbin</title>
-    <link rel="stylesheet" href="<%= Routes.static_path(@conn, "/css/app.css") %>"/>
+    <link rel="stylesheet" href="<%= Routes.static_path(@conn, "/assets/app.css") %>"/>
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
     <link href="https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;500;600;700;800&display=swap" rel="stylesheet">
-    <script defer type="text/javascript" src="<%= Routes.static_path(@conn, "/js/app.js") %>"></script>
+    <script defer type="text/javascript" src="<%= Routes.static_path(@conn, "/assets/app.js") %>"></script>
   </head>
   <body class="flex flex-col ">
     <header class="flex w-full justify-between items-center py-3 px-6">