Skip RBS rewriter when a file doesn't contain any RBS syntax

[amomchilov]

Before we waste the time parsing with Prism, running our RBS rewriting visitor, etc., we can do a quick scan through the file to see if it contains any RBS syntax at all. If not, we can just return it as it was.

This was already implemented into Tapioca, but would better live here in Spoom directly: Shopify/tapioca#2616