Fix crash when parsing activesupport's rbi

[dorner]

Core RBI files are allowed to have multiple overloading sig definitions. For example, this is taken from activesupport's rbi in sorbet-typed:

  # these are the sigs for Date- in the stdlib
  # https://github.com/sorbet/sorbet/blob/3910f6cfd9935c9b42e2135e32e15ab8a6e5b9be/rbi/stdlib/date.rbi#L373
  # note that if more sigs are added to sorbet you should replicate them here
  # check sorbet master: https://github.com/sorbet/sorbet/blob/master/rbi/stdlib/date.rbi
  sig {params(arg0: Numeric).returns(T.self_type)}
  sig {params(arg0: Date).returns(Rational)}
  # these sigs are added for activesupport users
  sig {params(arg0: ActiveSupport::Duration).returns(T.self_type)}
  def -(arg0); end

Parlour currently can't handle this and straight out crashes with the error: node after a sig must be a method definition

This PR fixes it so at least it doesn't crash. It doesn't go further by amalgamating all the sigs together into one (e.g. trying to figure out all the possible permutations of parameters and return values). Might be worth documenting this as a known bug.

[dorner]

Hmm... did not realize that sorbet-typed was no longer the place to get RBI files for gems. Using rbi-central doesn't have this issue. Not sure if this is worth merging then.