Allow to get list of non object methods

[qarmin]

1/3 of godotengine/godot-proposals#2344

This allow to get list of e.g. String methods by using:

ClassDB.get_variant_method_list(TYPE_STRING)

_ClassDB::get_variant_method_list is almost 1:1 copy of _ClassDB::get_method_list
also
Variant::get_method_list_by_type is just extracted code from Variant::get_method_list to be able not duplicate same code.

get_non_object_methods_list isn't the best name, so it should be renamed to something better

[YuriSizov]

get_non_object_methods_list isn't the best name, so it should be renamed to something better

Yeah... get_variant_method_list?

[Xrayez]

I've had issues with using Variant::Type in a method signatures while generating C# mono glue and while generating/validating RST documentation (see commit in VariantResource class in Goost). Not sure if that's still an issue in master, though.

If that's still an issue, the workaround is to use int over Variant::Type.

[Xrayez]

Looks like this cannot cause any issues because C# does not deal with Variant directly, and is a special case.

[akien-mga]

We discussed this in a PR meeting and @reduz suggests that this should not go in ClassDB because it's not part of its scope. This could maybe be implemented in @GlobalScope i.e. in a core/variant/ file directly (variant_internal.h? not sure).

[reduz]

I think we should create a singleton class in core_bind to expose Variant reflection to scripting, but I am not exactly sure what it should be called. Maybe TypeDB ?