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