Add a diagnostic for a bad re2 gem

When the system re2 library gets updated, it's often difficult and time-consuming to determine that a gem pristine re2 is needed. This gdk doctor diagnostic attempts to reproduce the seg fault that occurs as a result of https://github.com/mudge/re2/issues/43.

Testing notes

1. I upgraded re2 to v1.2 via brew upgrade re2
2. To downgrade to v1.1, you can brew edit re2, replace the contents with https://github.com/Homebrew/homebrew-core/blob/549a5c81c67e401fd3c75339157b3b936ebbbbed/Formula/re2.rb, and then brew reinstall re2.
3. To revert back to v1.2, you can restore https://github.com/Homebrew/homebrew-core/blob/40c227a5885561519827edecffd959f6316163d8/Formula/re2.rb and run brew upgrade re2.