Add TypeScript type alias declarations to scope

[devongovett]

Flow type aliases were scope tracked correctly, but TypeScript type aliases were not. This should fix that.

I wasn't quite sure where to add a test for this. Let me know if/where I should add one.

[benjamn]

@devongovett Thanks for working on this, and for using ast-types for react-docgen!

Can you either rebase against master now that I've fixed the tests, or open your branch so I can force-push to it?

If you have time to write a test, I would put it in test/typescript.ts, since this functionality is mostly TypeScript-specific, but you can model it after similar Scope tests in test/ecmascript.ts (using scope.lookupType and scope.declaresType instead of scope.lookup and scope.declares).

[devongovett]

Merged master and added tests.

[benjamn]

Now available in ast-types@0.12.4. Thanks again!