[ReactFlightWebpackPlugin] Add support for .mjs file extension

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.

Author: jennyscript

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

@@ -277,8 +277,8 @@ 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;
               }