Add more P/Invoke methods to get built-in entities

[copygirl]

I found myself needing the value of EcsDisabled, though unfortunately it's not exposed by flecs-cs. It would be nice if more (or all?) of the built-in entities were exposed such that we could use them without requiring them to be hard-coded or gotten dynamically from path/symbol using a world pointer. It would be even nicer if the bindgen could do this automatically.

[lithiumtoast]

Thanks for the feedback @copygirl!

I'll be looking into this next, this month.