GDScript: Check if method signature matches the parent

To guarantee polymorphism, a method signature must be compatible with
the parent. This checks if:

1. Return type is the same.
2. The subclass method takes at least the same amount of parameters.
3. The matching parameters have the same type.
4. If the subclass takes more parameters, all of the extra ones have a
default value.
5. If the superclass has default values, so must have the subclass.

There's a few test cases to ensure this holds up.

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