Tweak the pdf.scripting.js bundling, to improve overall consistency

This brings the new `pdf.scripting.js` bundling more in-line with the pre-existing handling for the  `pdf.js`/`pdf.worker.js` files:
 - Add a new `src/pdf.scripting.js` file as the entry-point for the build scripts.

 - Add the version/build numbers at the top of the *built* `pdf.scripting.js` files, since all other built files include that information given that it's often helpful to be able to easily determine the *exact* version.

 - Tweak the `createScriptingBundle` in the gulp-file, since it looks like a little bit too much copy-and-paste in the variable names.

src/pdf.scripting.js

@@ -0,0 +1,25 @@
+/* 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 { initSandbox } from "./scripting_api/initialization.js";
+
+/* eslint-disable-next-line no-unused-vars */
+const pdfjsVersion =
+  typeof PDFJSDev !== "undefined" ? PDFJSDev.eval("BUNDLE_VERSION") : void 0;
+/* eslint-disable-next-line no-unused-vars */
+const pdfjsBuild =
+  typeof PDFJSDev !== "undefined" ? PDFJSDev.eval("BUNDLE_BUILD") : void 0;
+
+export { initSandbox };