Use MethodInfo::get_compatibility_hash() to generate the hash for MethodBind::get_hash() and other GDExtension hash clean up

[dsnopek]

In PR #100674, MethodInfo::get_compatibility_hash() was added so that virtual methods could have compatibility hashes too.

The main goal of this PR is to change MethodBind::get_hash() to use MethodInfo::get_compatibility_hash() so that we have only one, consistent method for generating the hashes.

However, doing that turned up a handful of small bugs which this PR also fixes:

• There were 3 functions that had more default arguments than arguments - this PR removes those and adds entries to GDExtensionSpecialCompatHashes for them
• In order to try and prevent this from happening again, I've added a check in ClassDB::bind_method() that'll print out an error if there's more default arguments than arguments
• GetTypeInfo<GDExtensionConstPtr<T>>::VARIANT_TYPE and GetTypeInfo<GDExtensionPtr<T>>::VARIANT_TYPE is changed from Variant::NIL to the correct value of Variant::INT

These fixes have to be made for the MethodBind::get_hash() change to work, because, while the two methods generate the same exact hashes in the "correct case", they generate different hashes in the "incorrect cases" of the bugs above.

[akien-mga]

Thanks!