Add support for escape sequences in double quoted rbs strings

[johansenja]

I came across these earlier:

class A
  B = "a\nb"
end

class A
  B: "a\nb"
end

They were incompatible because rbs appeared to be handling double-quoted strings differently to ruby. This PR aims to bring support for newlines and other relatively common escape sequences

# rbs - existing workaround (for new lines at least)
class A
  B: "a
b"
end

[soutaro]

Thanks! 🙌