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_function.h

@@ -409,7 +409,8 @@ public:
 		ADDR_TYPE_STACK = 0,
 		ADDR_TYPE_CONSTANT = 1,
 		ADDR_TYPE_MEMBER = 2,
-		ADDR_TYPE_MAX = 3,
+		ADDR_TYPE_STATIC_VAR = 3,
+		ADDR_TYPE_MAX = 4,
 	};
 
 	enum FixedAddresses {