Add back esbuild-loader for .js files (go-gitea#31585)

The build currently seems to work fine without this, but in theory it
should fail because Fomantic is JavaScript. I feel more comfortable
having this loader declared.

Note: I did try using the [combined loader
config](https://github.com/privatenumber/esbuild-loader#-quick-setup)
but when I did that, vue files could not load, so I think this is the
best option.

webpack.config.js

@@ -139,6 +139,19 @@ export default {
           },
         },
       },
+      {
+        test: /\.js$/i,
+        exclude: /node_modules/,
+        use: [
+          {
+            loader: 'esbuild-loader',
+            options: {
+              loader: 'js',
+              target: 'es2020',
+            },
+          },
+        ],
+      },
       {
         test: /\.ts$/i,
         exclude: /node_modules/,