Go: remove the Go implementation in favor of bytecodealliance/go-modules

[Mossaka]

The new Go bindings generator is now actively developed and maintained in a separate repository: bytecodealliance/go-modules (Thanks to @ydnar!). I’d like to propose sunsetting the old CGo-based implementation in this repository.

My plan is to remove all Go-related source code and tests from this repo while keeping the Go subcommand in wit-bindgen. If users attempt to generate bindings with it, the tool will panic with a message directing them to use the new wit-bindgen-go CLI instead.

e.g.

> wit-bindgen tiny-go try.wit
Error: Go bindgen has been moved to a separate repository. Please visit https://github.com/bytecodealliance/go-modules for the new Go bindings generator `wit-bindgen-go`.

I will contribute all the existing runtime tests from this repo to the new repo, and @alexcrichton 's work on refactoring the test cases should help this process a lot, thank you!

Love to hear your thoughts on this one.

[alexcrichton]

(responded over on the associated PR)