GDScript: Fix extending abstract classes, forbid their construction

modules/gdscript/tests/scripts/analyzer/errors/abstract_class_instantiate.gd

@@ -0,0 +1,2 @@
+func test():
+	CanvasItem.new()

modules/gdscript/tests/scripts/analyzer/errors/abstract_class_instantiate.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Native class "CanvasItem" cannot be constructed as it is abstract.

modules/gdscript/tests/scripts/analyzer/errors/abstract_script_instantiate.gd

@@ -0,0 +1,9 @@
+class A extends CanvasItem:
+	func _init():
+		print('no')
+
+class B extends A:
+	pass
+
+func test():
+	B.new()

modules/gdscript/tests/scripts/analyzer/errors/abstract_script_instantiate.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Class "abstract_script_instantiate.gd::B" cannot be constructed as it is based on abstract native class "CanvasItem".

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