GDScript: Rework GDScriptUtilityFunctions macros

[dalexeev]

1. Rework macros for validating count and types of arguments. This makes the code easier to read and reduces the likelihood of accidental bugs. This also fixes a bug with implicit conversions don't work for some utility functions.
2. Rework function registration macros. Instead of several REGISTER_FUNC_* macros for different signature variations, there is now one REGISTER_FUNC macro and several shorthands for PropertyInfo/MethodInfo.

[Mickeon]

The side-effect of an actual enum for convert's argument is nice, but that's all of the input I can give.

[adamscott]

Seems ok, and seems to remove a lot of boilerplate code. I think that if the tests pass, it's a good sign to merge.

[Repiteo]

Thanks!