Add webpack file for legacy build.

This supports using the legacy PDF.js with webpack.

Co-authored-by: Stacey Beard <staceybeard10@gmail.com>

external/dist/legacy/webpack.mjs

@@ -0,0 +1,26 @@
+/* Copyright 2022 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+/* eslint-disable import/no-unresolved */
+
+import { GlobalWorkerOptions } from "./build/pdf.mjs";
+
+if (typeof window !== "undefined" && "Worker" in window) {
+  GlobalWorkerOptions.workerPort = new Worker(
+    new URL("./build/pdf.worker.mjs", import.meta.url),
+    { type: "module" }
+  );
+}
+
+export * from "./build/pdf.mjs";