GDScript: Add return type covariance and parameter type contravariance

[dalexeev]

• Closes Support covariant return types for overriding functions godot-proposals#7450.
• Supersedes Support covariant return types of overriding functions #80457.

CC @bitbrain @adamscott @vonagam

[bitbrain]

@dalexeev thank you so much for this! I remember from our discussion that in the codebase we have several places where we need to know if "something is a covariant of something else":

1. would it make sense to create a dedicated method for it (used by the analyzer) rather than having it baked into the analyzer?
2. are there currently other places in the codebase that could benefit from this method? I remember you mentioned that there are multiple places where we may require to check for "is type X of type Y"

[dalexeev]

would it make sense to create a dedicated method for it

Currently no, we are close to feature freeze and release, this is not the time for unreasonable changes. But I have two draft branches: one is an attempt to refactor is_type_compatible() method and the other is an attempt to refactor DataType class.

[dalexeev]

• See also Don't allow weaker return type in overriden method #79765.

[YuriSizov]

Thanks!