Refactored type and property helpers

[kraenhansen]

What, How & Why?

This refactors the type and property helpers into multiple files.

I want to move away from the "internal-pattern" to get rid of the circular reference warnings, which I somewhat started here and I'll have to do a separate (massive) PR later to complete that.

At some point, I think it'll also be nice to move some of the functions introduced in https://github.com/realm/realm-js/pull/6613 into type specific property helpers.