* chore: update relevant Node.js versions from 8 to 10
* chore: remove node6 and node8 folders from puppeteer-firefox ci
* fix: loosen definition for proc.stdio
* fix: update typescript version used in npm run test-types
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.