Add support for static variables in GDScript

Which allows editable data associated with a particular class instead of
the instance. Scripts with static variables are kept in memory
indefinitely unless the `@static_unload` annotation is used or the
`static_unload()` method is called on the GDScript.

If the custom function `_static_init()` exists it will be called when
the class is loaded, after the static variables are set.

modules/gdscript/gdscript_codegen.h

@@ -44,6 +44,7 @@ public:
 			CLASS,
 			MEMBER,
 			CONSTANT,
+			STATIC_VARIABLE,
 			LOCAL_VARIABLE,
 			FUNCTION_PARAMETER,
 			TEMPORARY,