LibJS: Implement Temporal.PlainYearMonth.prototype[@@toStringTag]

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/d8e835d22f1
Pull-request: https://github.com/SerenityOS/serenity/pull/9273
Reviewed-by: https://github.com/IdanHo ✅

Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp

@@ -18,6 +18,11 @@ PlainYearMonthPrototype::PlainYearMonthPrototype(GlobalObject& global_object)
 void PlainYearMonthPrototype::initialize(GlobalObject& global_object)
 {
     Object::initialize(global_object);
+
+    auto& vm = this->vm();
+
+    // 9.3.2 Temporal.PlainYearMonth.prototype[ @@toStringTag ], https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth.prototype-@@tostringtag
+    define_direct_property(*vm.well_known_symbol_to_string_tag(), js_string(vm.heap(), "Temporal.PlainYearMonth"), Attribute::Configurable);
 }
 
 }

Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.@@toStringTag.js

@@ -0,0 +1,3 @@
+test("basic functionality", () => {
+    expect(Temporal.PlainYearMonth.prototype[Symbol.toStringTag]).toBe("Temporal.PlainYearMonth");
+});