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

Author: https://github.com/IdanHo
Commit: 94e1324a67
Pull-request: https://github.com/SerenityOS/serenity/pull/8870
Reviewed-by: https://github.com/linusg

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

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