LibJS: Implement the Temporal.PlainMonthDay constructor

And the simple Temporal.PlainMonthDay.prototype getters, so that the
constructed Temporal.PlainMonthDay may actually be validated.

Libraries/LibJS/Tests/builtins/Temporal/PlainMonthDay/PlainMonthDay.from.js

@@ -0,0 +1,104 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.PlainMonthDay.from).toHaveLength(1);
+    });
+
+    test("fields object argument", () => {
+        const object = {
+            month: 7,
+            day: 6,
+        };
+        const plainMonthDay = Temporal.PlainMonthDay.from(object);
+        expect(plainMonthDay.monthCode).toBe("M07");
+        expect(plainMonthDay.day).toBe(6);
+    });
+
+    test("from month day string", () => {
+        const plainMonthDay = Temporal.PlainMonthDay.from("--07-06");
+        expect(plainMonthDay.monthCode).toBe("M07");
+        expect(plainMonthDay.day).toBe(6);
+    });
+
+    test("from date time string", () => {
+        const plainMonthDay = Temporal.PlainMonthDay.from("2021-07-06T23:42:01");
+        expect(plainMonthDay.monthCode).toBe("M07");
+        expect(plainMonthDay.day).toBe(6);
+    });
+
+    test("compares calendar name in month day string in lowercase", () => {
+        const values = [
+            "02-10[u-ca=iso8601]",
+            "02-10[u-ca=isO8601]",
+            "02-10[u-ca=iSo8601]",
+            "02-10[u-ca=iSO8601]",
+            "02-10[u-ca=Iso8601]",
+            "02-10[u-ca=IsO8601]",
+            "02-10[u-ca=ISo8601]",
+            "02-10[u-ca=ISO8601]",
+        ];
+
+        for (const value of values) {
+            expect(() => {
+                Temporal.PlainMonthDay.from(value);
+            }).not.toThrowWithMessage(
+                RangeError,
+                "MM-DD string format can only be used with the iso8601 calendar"
+            );
+        }
+    });
+});
+
+describe("errors", () => {
+    test("missing fields", () => {
+        expect(() => {
+            Temporal.PlainMonthDay.from({});
+        }).toThrowWithMessage(TypeError, "Required property day is missing or undefined");
+        expect(() => {
+            Temporal.PlainMonthDay.from({ month: 1 });
+        }).toThrowWithMessage(TypeError, "Required property day is missing or undefined");
+        expect(() => {
+            Temporal.PlainMonthDay.from({ day: 1 });
+        }).toThrowWithMessage(TypeError, "Required property month is missing or undefined");
+    });
+
+    test("invalid month day string", () => {
+        expect(() => {
+            Temporal.PlainMonthDay.from("foo");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+    });
+
+    test("string must not contain a UTC designator", () => {
+        expect(() => {
+            Temporal.PlainMonthDay.from("2021-07-06T23:42:01Z");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+    });
+
+    test("extended year must not be negative zero", () => {
+        expect(() => {
+            Temporal.PlainMonthDay.from("-000000-01-01");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+    });
+
+    test("can only use iso8601 calendar with month day strings", () => {
+        expect(() => {
+            Temporal.PlainMonthDay.from("02-10[u-ca=iso8602]");
+        }).toThrowWithMessage(RangeError, "Invalid calendar identifier 'iso8602'");
+
+        expect(() => {
+            Temporal.PlainMonthDay.from("02-10[u-ca=ladybird]");
+        }).toThrowWithMessage(RangeError, "Invalid calendar identifier 'ladybird'");
+    });
+
+    test("doesn't throw non-iso8601 calendar error when using a superset format string such as DateTime", () => {
+        // NOTE: This will still throw, but only because "ladybird" is not a recognised calendar, not because of the string format restriction.
+        try {
+            Temporal.PlainMonthDay.from("2023-02-10T22:56[u-ca=ladybird]");
+        } catch (e) {
+            expect(e).toBeInstanceOf(RangeError);
+            expect(e.message).not.toBe(
+                "MM-DD string format can only be used with the iso8601 calendar"
+            );
+            expect(e.message).toBe("Invalid calendar identifier 'ladybird'");
+        }
+    });
+});