GDScript: Do not allow standalone lambdas

They cannot be accessed in this case, so an error is shown to avoid
misleading the uses, especially in case of named lambdas.

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

@@ -0,0 +1,3 @@
+func test():
+	func standalone():
+		print("can't be accessed")

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

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Standalone lambdas cannot be accessed. Consider assigning it to a variable.