Add a keyword for abstract classes in GDScript

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

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

@@ -0,0 +1,10 @@
+extends RefCounted
+
+const AbstractScript = preload("./construct_abstract_script.notest.gd")
+
+abstract class AbstractClass:
+	pass
+
+func test():
+	var _a := AbstractScript.new()
+	var _b := AbstractClass.new()

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

@@ -0,0 +1,3 @@
+GDTEST_ANALYZER_ERROR
+>> ERROR at line 9: Cannot construct abstract class "AbstractScript".
+>> ERROR at line 10: Cannot construct abstract class "AbstractClass".

modules/gdscript/tests/scripts/analyzer/errors/construct_abstract_script.notest.gd

@@ -0,0 +1 @@
+abstract class_name AbstractScript

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())

modules/gdscript/tests/scripts/parser/errors/duplicate_abstract.gd

@@ -0,0 +1,7 @@
+extends RefCounted
+
+abstract abstract class A:
+	pass
+
+func test():
+	pass

modules/gdscript/tests/scripts/parser/errors/duplicate_abstract.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Expected "class_name", "extends", or "class" after "abstract".