Homebrew: Selfdestruct

The name is very open to change. I only really went with something so drastic so that folks are unlikely to ever stumble onto it accidentally. Ideally we could do with adding a confirmation prompt that requests a “yes” before continuing and then the name could be scaled back to something less drastic. 

I’m not overly keen on “unbrew” as that feels like something users could stumble onto trying to remove formula, and ending up eradicating the entire Homebrew install.

Script taken from Steve Benner’s excellent work [here](https://gist.github.com/SteveBenner/11254428) and tweaked a teeny, tiny bit to fit inside Homebrew’s formatting. My major changes are lines 9, 10, 176 & 177. Everything else, aside a couple of squished code comments, is identical to Steve’s script.

This PR relates to [#28174](https://github.com/Homebrew/homebrew/issues/28174), amongst many other issues. Happy to work on this as much as necessary/desired.
So ``` #{ENV['HOME']}/Library/Caches/Homebrew ``` can be culled into ``` #{HOMEBREW_CACHE} ```?