GDExtension support

[dugramen]

Requires a lot

• Redirect all res:// paths in .gdextension files
• Redirect all relative paths in .gdextension files
• Locate and load extensions with GDExtensionManager.load_extension()

GDExtension classes are available in ClassDB methods, but not gdscript's global scope

• Detect added classes by diffing ClassDB.get_class_list() before and after loading extensions. Might have to manually unload and reload extensions
• Codegen wrapper classes for all detected classes using ClassDB
  • _init() calls ClassDB.instantiate and stores a reference to the instance
  • All other methods and properties should correspond to that instance
  • _static_init() instantiates 1 shared instance just for static calls, since instances can access static methods
• Build wrapper classes into 1 big wrapper class file, and include in processed gdscripts using the workaround Add all class_names as manual preloads to all processed gdscripts #9