Changes the `<godot/gdextension_interface.h>` include to simply
`<gdextension_interface.h>`.
Refactor and better document the SCons and CMake logic around setting
the paths to the header and API JSON file.
Non-exhaustive list of case-sensitive renames:
GDExtension -> GDNative
GDNATIVE -> GDEXTENSION
gdextension -> gdnative
ExtensionExtension -> Extension (for where there was GDNativeExtension)
EXTENSION_EXTENSION -> EXTENSION (for where there was GDNATIVE_EXTENSION)
gdnlib -> gdextension
gdn_interface -> gde_interface
gdni -> gde_interface
This makes sure custom constructors are always called on extension
classes. However, note that constructors should not take any parameters,
since Godot doesn't support that. Parameters are ignore in memnew macro.
Use memnew(MyClass()) instead of memnew(MyClass) since it now needs a
value instead of a class name. This macro calls MyClass::_new() (define
in GDCLASS macro) which ultimately calls Godot to create the object,
ensuring that both the Godot and the extension instances are created.
Non Godot classes (that don't derive godot::Object) are constructed as
usual an can have parameters.
memdelete is also changed for the same reason, as it needs to destroy
the Godot object as well, and that automatically frees the bound
extension instance.
