Move the GenericScripting class to its own file, such that it can be used in the Chromium-extension

While it's not entirely clear to me that it's ultimately desirable to use the `pdf.sandbox.js` in the Chromium-extension, given that the MOZCENTRAL-build uses `pdf.scripting.js` directly in a *custom* sandbox, the current state isn't that great since setting `enableScripting = true` with the Chromium-extension will currently fail completely.

Hence this patch, which should at least unbreak things for now.

web/generic_scripting.js

@@ -0,0 +1,45 @@
+/* Copyright 2020 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.
+ */
+
+import { AppOptions } from "./app_options.js";
+import { loadScript } from "pdfjs-lib";
+
+class GenericScripting {
+  constructor() {
+    this._ready = loadScript(
+      AppOptions.get("sandboxBundleSrc"),
+      /* removeScriptElement = */ true
+    ).then(() => {
+      return window.pdfjsSandbox.QuickJSSandbox();
+    });
+  }
+
+  async createSandbox(data) {
+    const sandbox = await this._ready;
+    sandbox.create(data);
+  }
+
+  async dispatchEventInSandbox(event) {
+    const sandbox = await this._ready;
+    sandbox.dispatchEvent(event);
+  }
+
+  async destroySandbox() {
+    const sandbox = await this._ready;
+    sandbox.nukeSandbox();
+  }
+}
+
+export { GenericScripting };