Add Windows support

[Matthew-Benson]

This is partially scaffolded out, but there's a long way to go. This would be a big win and improve hermetacity
vs writing system bash and powershell/cmd for genrules that need to support windows and scripts that need to be OS independent.

Today, Fish for windows requires Cygwin. See https://fishshell.com/ for details. This may change for the Rust build of Fish and that may be much easier to support.

• Need toolchain provider info to pick a fish.exe file.
• Need to make the fish wrapper base a powershell script rather than bash.