LibJS: Implement Temporal.Calendar.prototype.inLeapYear

Author: https://github.com/IdanHo
Commit: c9ae7e1af1
Pull-request: https://github.com/SerenityOS/serenity/pull/8961
Reviewed-by: https://github.com/linusg ✅

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

@@ -0,0 +1,13 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.Calendar.prototype.inLeapYear).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const calendar = new Temporal.Calendar("iso8601");
+        const date = new Temporal.PlainDate(2021, 7, 23);
+        expect(calendar.inLeapYear(date)).toBeFalse();
+        const leapDate = new Temporal.PlainDate(2020, 7, 23);
+        expect(calendar.inLeapYear(leapDate)).toBeTrue();
+    });
+});