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/completeness.sh

@@ -0,0 +1,7 @@
+set -e
+
+total=`git grep '  \* \[' README.md| wc -l`
+complete=`git grep '  \* \[x' README.md | wc -l`
+ratio=`echo "$complete / $total * 100" | bc -l`
+printf "%.2f%%\n" $ratio
+