refactor(dev-scripts): Simplify resolve.extensions

There doesn't appear to be any reason not to include the '.ts'
in resolve.extensions even for pure-JS plugins, but it is
_necessary_ to include it in TS plugins.

Since the default value for resolve.extensions is
    ['.js', '.json', '.wasm']
set it to
    ['.ts', '.js', '.json', '.wasm']
which gives priority to TS, then JS, then the other default
extensions.

Also add a helpful comment explaining the purpose of
resolve.fallback.

plugins/dev-scripts/config/webpack.config.js

@@ -93,9 +93,10 @@ module.exports = (env) => {
       clean: true,
     },
     resolve: {
-      extensions: ['.ts', '.js'].filter(
-        (ext) => isTypescript || !ext.includes('ts'),
-      ),
+      extensions: ['.ts', '.js', '.json', '.wasm'],
+      // Some deps may require node.js core modules.  Tell node.js what
+      // polyfills to use for them when building for non-node.js targets
+      // (Or to ignore them if the fallback is false.)
       fallback: {
         util: false,
       },