Add a command to prune all generated/built files from a checkout

[ivanlei]

What is the Problem Being Solved?

If a build fails (or even if it doesn't), the checkout will have numerous output files, node_modules / directories, built binary packages, etc. A command to prune all generated/built files from a checkout will help to minimize the chance that build artifacts on disk prevent further builds from succeeding.

Description of the Design

The command should either add to yarn clean or be some sort of yarn super-duper-clean

Security Considerations

Should be none

Scaling Considerations

Should be none

Test Plan

[arirubinstein]

perhaps git clean -dfx ?

[mhofman]

Indeed, I often run git clean -xfd && git submodule foreach --recursive git clean -xfd