GDScript: Add abstract methods

Co-authored-by: ryanabx <ryanbrue@hotmail.com>

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

@@ -0,0 +1,28 @@
+abstract class AbstractClass:
+	abstract func some_func()
+
+class ImplementedClass extends AbstractClass:
+	func some_func():
+		pass
+
+abstract class AbstractClassAgain extends ImplementedClass:
+	abstract func some_func()
+
+class Test1:
+	abstract func some_func()
+
+class Test2 extends AbstractClass:
+	pass
+
+class Test3 extends AbstractClassAgain:
+	pass
+
+class Test4 extends AbstractClass:
+	func some_func():
+		super()
+
+	func other_func():
+		super.some_func()
+
+func test():
+	pass