Set up spec for the rbi

[hdoan741]

There should be a way to test the rbi so that reviewing PR is easier. I expect it would be:

• Set up sorbet in this repo
• Set up structure to add testcase & output from srb tc

When people add rbi, they should also add test cases & expected output

[connorshea]

I was curious how DefinitelyTyped does this, and it looks like they have a tests file for every package that includes some basic code for the package, to make sure it passes static type checking.

e.g. actioncable/index.d.ts and actioncable-tests.ts

That seems like the simplest solution to me.

[jasnow]

Can we run a rbi-based "lint" as a first step at testing?