Add Sorbet/ForbidSig and Sorbet/ForbidSigWithRuntime

[Morriar]

So people can choose to allow/disallow the different shapes of sigs:

sig { void }
def foo; end

T::Sig.sig { void }
def foo; end

T::Sig::WithoutRuntime.sig { void }
def foo; end

[andyw8]

I think we can help educate people about why this is, e.g.

Suggested change

	MSG = "Do not use `T::Sig`."
	MSG = "Prefer the RBS signature comment syntax over `T::Sig`."

[Morriar]

I didn't on purpose, same reason there is not autocorrect.

I wonder if we should leverage the Details config option for this? 🤔

[Morriar]

Actually I even removed the hint from ForbidSigWithoutRuntime message: a5ac3e4