Add GDExtension support to Script

* Ability to create script languages from GDExtension
* Some additions to gdnative_extension.h to make this happen
* Moved the GDExtension binder to core

This now allows creating scripting languages from GDExtension, with the same ease as if it was a module. It replaces the old PluginScript from Godot 3.x.
Warning: GodotCPP will need to be updated to support this (it may be a bit of work as ScriptInstance needs to be created over there again).

core/extension/SCsub

@@ -2,6 +2,12 @@
 
 Import("env")
 
+import make_wrappers
+from platform_methods import run_in_subprocess
+
+env.CommandNoCache(["ext_wrappers.gen.inc"], "make_wrappers.py", run_in_subprocess(make_wrappers.run))
+
+
 env_extension = env.Clone()
 
 env_extension.add_source_files(env.core_sources, "*.cpp")