LibJS: Handle PlainMonthDay in the Calendar.prototype getters

Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.month.js

@@ -9,3 +9,16 @@ describe("correct behavior", () => {
         expect(calendar.month(date)).toBe(7);
     });
 });
+
+describe("errors", () => {
+    test("argument must not be a Temporal.PlainMonthDay object", () => {
+        const calendar = new Temporal.Calendar("iso8601");
+        const plainMonthDay = new Temporal.PlainMonthDay(7, 6);
+        expect(() => {
+            calendar.month(plainMonthDay);
+        }).toThrowWithMessage(
+            TypeError,
+            "Accessing month of PlainMonthDay is ambiguous, use monthCode instead"
+        );
+    });
+});