Move https://github.com/marco-c/firefox-code-coverage into this repo

https://github.com/marco-c/firefox-code-coverage simply contains a script for developers to locally generate coverage reports.
It's duplicating some of the code in this repo, it'd be much better if we moved it here.

When we do this, we should update https://wiki.developer.mozilla.org/en-US/docs/Mozilla/Testing/Measuring_Code_Coverage_on_Firefox.