LibJS: Implement Temporal.PlainYearMonth.prototype.daysInMonth

Author: https://github.com/linusg
Commit: 703eb1f7b4
Pull-request: https://github.com/SerenityOS/serenity/pull/9273
Reviewed-by: https://github.com/IdanHo ✅

Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.daysInMonth.js

@@ -0,0 +1,14 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const plainYearMonth = new Temporal.PlainYearMonth(2021, 7);
+        expect(plainYearMonth.daysInMonth).toBe(31);
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.PlainYearMonth object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainYearMonth.prototype, "daysInMonth", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainYearMonth");
+    });
+});