GDScript: Fix extending abstract classes, forbid their construction

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

@@ -0,0 +1,12 @@
+class A extends CanvasItem:
+	func _init():
+		pass
+
+class B extends A:
+	pass
+
+class C extends CanvasItem:
+	pass
+
+func test():
+	print('ok')

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

@@ -0,0 +1,2 @@
+GDTEST_OK
+ok