feat: introduce puppeteer-firefox (#3628)

This adds a proof-of-concept of `puppeteer-firefox`.
This consists of two parts:
- `//experimental/juggler` - patches to apply to Firefox.
- `//experimental/puppeteer-firefox` - front-end code to
be merged with Puppeteer.

As things become more stable, we'll gradually move it out of
the experimental folder.

experimental/puppeteer-firefox/misc/00-puppeteer-prefs.js

@@ -0,0 +1,3 @@
+// Any comment. You must start the file with a single-line comment!
+pref("general.config.filename", "puppeteer.cfg");
+pref("general.config.obscure_value", 0);