Add a keyword for abstract classes in GDScript

Co-authored-by: Danil Alexeev <danil@alexeev.xyz>

modules/gdscript/tests/scripts/analyzer/features/extend_abstract_class.gd

@@ -8,5 +8,12 @@ class B extends A:
 class C extends CanvasItem:
 	pass
 
+abstract class X:
+	pass
+
+class Y extends X:
+	func test() -> String:
+		return "ok"
+
 func test():
-	print('ok')
+	print(Y.new().test())