Convert to TypeScript

[gitKrystan]

This is a bit of a yak shave.

1. This codemod currently does not support legacy decorators, but we have users stuck on Ember classic that use legacy decorators (specifically, decorators in object literals).
2. The currently-used Flow parser does not support legacy decorators, necessitating a switch to the babel parser. I did a POC spike on switching to this parser, and while it does parse the legacy decorators, there is enough of a shift in the AST format that it causes many of the existing tests to fail. In my spike, I started fixing the code to make these tests pass, but without TypeScript + good AST types, I don't have 100% confidence I'm not breaking things, and working on it is a bit like this gif:
   

See also: gitKrystan#1, gitKrystan#2

[gitKrystan]

Approved by @wycats via Telegram