Remove duplicate classes

[CedNaru]

We had many Godot classes (mostly enums) that we reimplemented back in Godot 3 and used all across our Kotlin modules.
Since Godot 4, they are all part of the api and are generated automatically. We never properly modified our code to reflect that.
It was done progressively over 1 year of changes. I simply decided to remove the last ones:
GodotError and VariantType.

GodotError was simply deleted and replaced by the correct generated one where necessary.
VariantType is a bit special, as we are using it internally. On top of that, the "true" VariantType is occasionally used within the Godot API (example: TileSet::get_custom_data_layer_type). Like GodotError, I replaced the VariantType used in the API generation when it's a parameter or return type.

To avoid confusion with the one we use to write/read the buffer (and also used in the internals of the generated API), I changed its name to VariantParser, which is also a name used in Godot C++ code, that I think is fitting.