Add support for modern ECMAScript class features

With ESLint 8 we should now finally be able to start using modern `class` features, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Public_class_fields and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields

However, while both ESLint and Acorn now support this, it unfortunately turns out that Escodegen (which we use during building) still lack the necessary support. Looking at https://github.com/estools/escodegen there's not been any updates since last year, and there's also open PRs adding support for these new `class` features.

To avoid blocking usage of these `class` features in the PDF.js code-base, in particular *private* fields/methods, this patch thus proposes that we (hopefully temporarily) switch to an `escodegen` fork that has the necessary support; please see https://www.npmjs.com/package/@javascript-obfuscator/escodegen

While I have no reason to doubt the security of the `escodegen` fork, this patch nonetheless pins the version number. Furthermore, I've also diffed the output of the two `.js`-files in this forked package against the original files without finding anything that looks immediately "dangerous".

test/types/tsconfig.json

@@ -6,14 +6,14 @@
     "moduleResolution": "node",
     "emitDecoratorMetadata": true,
     "experimentalDecorators": true,
-    "target": "es5",
-    "module": "es2015",
+    "target": "ESNext",
+    "module": "ESNext",
     "baseUrl": "./",
     "strict": true,
     "types": [],
     "lib": [
-      "es2017",
-      "dom"
+      "ESNext",
+      "DOM"
     ],
     "paths": {
       "pdfjs-dist": ["../../build/typestest"],