Stop exposing the URL polyfill in the global scope

This moves/exposes the `URL` polyfill similarily to the existing `ReadableStream` polyfill, rather than exposing it globally, to avoid interfering with any "outside" code.
Both the `URL` and `ReadableStream` polyfills are now exposed on the `pdfjsLib` object, such that they are accessible to the viewer components.
Furthermore, the `no-restricted-globals` ESLint rule is also enabled to prevent accidental usage of the native `URL`/`ReadableStream` implementations directly in the `src/` and `web/` folders; see also https://eslint.org/docs/rules/no-restricted-globals

Addresses the remaining TODO in https://github.com/mozilla/pdf.js/projects/6

.eslintrc

@@ -98,6 +98,7 @@
     "no-catch-shadow": "error",
     "no-delete-var": "error",
     "no-label-var": "error",
+    "no-restricted-globals": "off",
     "no-shadow-restricted-names": "error",
     "no-shadow": "off",
     "no-undef-init": "error",