This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
The GDVIRTUAL_NATIVE_PTR did not declare the correct GDNativeConstPtr
template, resulting in "void*" being used as it's type info in both the
documentation and the extension API dump.
Fabio Alessandrelli
Yuri Sizov
Hugo Locurcio