refactor: module type and type member refactoring

[arendjr]

Summary

This introduces the TypeData::Module variant, which I intend to use for declare module statements in .d.ts files.

However, the Module type contains TypeMember, which should now be able to hold arbitrary type definitions. I already felt TypeMember was implemented a bit awkwardly, causing some unnecessary duplication, so I've taken the opportunity to refactor it. Now all type members (whether they be object members, class members, or module members) refer to registered types, which can be of any kind of TypeData.

This means we don't make an explicit distinction between properties and methods anymore, the difference is simply whether the reference points to a function or not. Optionality is also no longer an explicit field; it's simply a matter of the referenced type being a union with undefined.

Another side-effect is that our snapshots now make it easier to inspect inferred method signatures, since they've become an explicit part of the registered types.

Test Plan

Tests are updated.