LibJS: Implement Temporal.PlainDateTime.prototype.inLeapYear

Author: https://github.com/IdanHo
Commit: 8f9b4a5ea6
Pull-request: https://github.com/SerenityOS/serenity/pull/9097
Reviewed-by: https://github.com/linusg ✅

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

@@ -0,0 +1,14 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const plainDateTime = new Temporal.PlainDateTime(2021, 7, 30);
+        expect(plainDateTime.inLeapYear).toBeFalse();
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Temporal.PlainDateTime object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainDateTime.prototype, "inLeapYear", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainDateTime");
+    });
+});