Do not rely on return type for attr_writer RBS signatures

[Morriar]

The current RBS translation mechanism for attr_writers relies on the return type of its signature. However, the return type can't always be trusted as both these syntax are valid for Sorbet:

sig { params(x: Integer).returns(Integer) }
attr_writer :x

sig { params(y: Integer).void }
attr_writer :y

Instead, we should rely on the first parameter which we can be sure to find since Sorbet requires it.

[amomchilov]

Subtle, good catch

[paracycle]

Makes sense!