[ReactFlightWebpackPlugin] Add support for .mjs file extension (facebook#33028)

## Summary
Our builds generate files with a `.mjs` file extension. These are
currently filtered out by `ReactFlightWebpackPlugin` so I am updating it
to support this file extension.

This fixes facebook#33155

## How did you test this change?
I built the plugin with this change and used `yalc` to test it in my
project. I confirmed the expected files now show up in
`react-client-manifest.json`

Author: jennyscript

packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js

@@ -277,8 +277,14 @@ export default class ReactFlightWebpackPlugin {
             chunkGroup.chunks.forEach(function (c) {
               // eslint-disable-next-line no-for-of-loops/no-for-of-loops
               for (const file of c.files) {
-                if (!file.endsWith('.js')) return;
-                if (file.endsWith('.hot-update.js')) return;
+                if (!(file.endsWith('.js') || file.endsWith('.mjs'))) {
+                  return;
+                }
+                if (
+                  file.endsWith('.hot-update.js') ||
+                  file.endsWith('.hot-update.mjs')
+                )
+                  return;
                 chunks.push(c.id, file);
                 break;
               }