Add support for top-level bind in sigs

[Morriar]

Summary

Add support for Sorbet’s top-level bind signature modifier:

sig { bind(Foo).params(value: String).returns(Symbol) }

Previously, the parser silently discarded bind(Foo).

• store the bind type on RBI::Sig

• parse and print bind(...) when round-tripping RBI

• normalize braceless shape bind types:

  sig { bind("foo" => Integer).void }

  becomes:

  sig { bind({"foo" => Integer}).void }

• include bind types in signature equality and merge behavior

• preserve bind types when rewriting attributes into methods

• share type-argument extraction between bind and returns

RBS output intentionally omits top-level bind. Sorbet uses it to set the type of self while checking the method body, and RBS has no equivalent method-level self binding.

[paracycle]

I actually didn't know that you could do a top-level bind call like this either.