pdfjs: Add some comments to polyfills

qutebrowser/javascript/pdfjs_polyfills.js

@@ -1,7 +1,14 @@
 /* eslint-disable strict */
 /* (this file gets used as a snippet) */
 
+/*
+SPDX-FileCopyrightText: Florian Bruhin (The Compiler) <mail@qutebrowser.org>
+SPDX-License-Identifier: GPL-3.0-or-later
+*/
+
 (function() {
+    // Chromium 119 / QtWebEngine 6.8
+    // https://caniuse.com/mdn-javascript_builtins_promise_withresolvers
     if (typeof Promise.withResolvers === "undefined") {
         Promise.withResolvers = function() {
             let resolve, reject