[GDScript] Add `StringName` support to `@GDScript.len`

modules/gdscript/doc_classes/@GDScript.xml

@@ -148,7 +148,7 @@
 			<return type="int" />
 			<param index="0" name="var" type="Variant" />
 			<description>
-				Returns the length of the given Variant [param var]. The length can be the character count of a [String], the element count of any array type or the size of a [Dictionary]. For every other Variant type, a run-time error is generated and execution is stopped.
+				Returns the length of the given Variant [param var]. The length can be the character count of a [String] or [StringName], the element count of any array type, or the size of a [Dictionary]. For every other Variant type, a run-time error is generated and execution is stopped.
 				[codeblock]
 				a = [1, 2, 3, 4]
 				len(a) # Returns 4

modules/gdscript/gdscript_utility_functions.cpp

@@ -470,7 +470,8 @@ struct GDScriptUtilityFunctionsDefinitions {
 	static inline void len(Variant *r_ret, const Variant **p_args, int p_arg_count, Callable::CallError &r_error) {
 		VALIDATE_ARG_COUNT(1);
 		switch (p_args[0]->get_type()) {
-			case Variant::STRING: {
+			case Variant::STRING:
+			case Variant::STRING_NAME: {
 				String d = *p_args[0];
 				*r_ret = d.length();
 			} break;