LibJS: Implement the Temporal.PlainYearMonth constructor

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

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

@@ -0,0 +1,130 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.PlainYearMonth.from).toHaveLength(1);
+    });
+
+    test("PlainYearMonth instance argument", () => {
+        const plainYearMonth_ = new Temporal.PlainYearMonth(2021, 7);
+        const plainYearMonth = Temporal.PlainYearMonth.from(plainYearMonth_);
+        expect(plainYearMonth.year).toBe(2021);
+        expect(plainYearMonth.month).toBe(7);
+        expect(plainYearMonth.monthCode).toBe("M07");
+        expect(plainYearMonth.daysInYear).toBe(365);
+        expect(plainYearMonth.daysInMonth).toBe(31);
+        expect(plainYearMonth.monthsInYear).toBe(12);
+        expect(plainYearMonth.inLeapYear).toBeFalse();
+    });
+
+    test("fields object argument", () => {
+        const object = {
+            year: 2021,
+            month: 7,
+        };
+        const plainYearMonth = Temporal.PlainYearMonth.from(object);
+        expect(plainYearMonth.year).toBe(2021);
+        expect(plainYearMonth.month).toBe(7);
+        expect(plainYearMonth.monthCode).toBe("M07");
+        expect(plainYearMonth.daysInYear).toBe(365);
+        expect(plainYearMonth.daysInMonth).toBe(31);
+        expect(plainYearMonth.monthsInYear).toBe(12);
+        expect(plainYearMonth.inLeapYear).toBeFalse();
+    });
+
+    test("from year month string", () => {
+        const plainYearMonth = Temporal.PlainYearMonth.from("2021-07");
+        expect(plainYearMonth.year).toBe(2021);
+        expect(plainYearMonth.month).toBe(7);
+        expect(plainYearMonth.monthCode).toBe("M07");
+    });
+
+    test("from date time string", () => {
+        const plainYearMonth = Temporal.PlainYearMonth.from("2021-07-06T23:42:01");
+        expect(plainYearMonth.year).toBe(2021);
+        expect(plainYearMonth.month).toBe(7);
+        expect(plainYearMonth.monthCode).toBe("M07");
+        expect(plainYearMonth.daysInYear).toBe(365);
+        expect(plainYearMonth.daysInMonth).toBe(31);
+        expect(plainYearMonth.monthsInYear).toBe(12);
+        expect(plainYearMonth.inLeapYear).toBeFalse();
+    });
+
+    test("compares calendar name in year month string in lowercase", () => {
+        const values = [
+            "2023-02[u-ca=iso8601]",
+            "2023-02[u-ca=isO8601]",
+            "2023-02[u-ca=iSo8601]",
+            "2023-02[u-ca=iSO8601]",
+            "2023-02[u-ca=Iso8601]",
+            "2023-02[u-ca=IsO8601]",
+            "2023-02[u-ca=ISo8601]",
+            "2023-02[u-ca=ISO8601]",
+        ];
+
+        for (const value of values) {
+            expect(() => {
+                Temporal.PlainYearMonth.from(value);
+            }).not.toThrowWithMessage(
+                RangeError,
+                "YYYY-MM string format can only be used with the iso8601 calendar"
+            );
+        }
+    });
+});
+
+describe("errors", () => {
+    test("missing fields", () => {
+        expect(() => {
+            Temporal.PlainYearMonth.from({});
+        }).toThrowWithMessage(TypeError, "Required property year is missing or undefined");
+        expect(() => {
+            Temporal.PlainYearMonth.from({ year: 0 });
+        }).toThrowWithMessage(TypeError, "Required property month is missing or undefined");
+        expect(() => {
+            Temporal.PlainYearMonth.from({ month: 1 });
+        }).toThrowWithMessage(TypeError, "Required property year is missing or undefined");
+    });
+
+    test("invalid year month string", () => {
+        expect(() => {
+            Temporal.PlainYearMonth.from("foo");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+    });
+
+    test("string must not contain a UTC designator", () => {
+        expect(() => {
+            Temporal.PlainYearMonth.from("2021-07-06T23:42:01Z");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+    });
+
+    test("extended year must not be negative zero", () => {
+        expect(() => {
+            Temporal.PlainYearMonth.from("-000000-01");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+        expect(() => {
+            Temporal.PlainYearMonth.from("−000000-01"); // U+2212
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+    });
+
+    test("can only use iso8601 calendar with year month strings", () => {
+        expect(() => {
+            Temporal.PlainYearMonth.from("2023-02[u-ca=iso8602]");
+        }).toThrowWithMessage(RangeError, "Invalid calendar identifier 'iso8602'");
+
+        expect(() => {
+            Temporal.PlainYearMonth.from("2023-02[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.PlainYearMonth.from("2023-02-10T22:57[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'");
+        }
+    });
+});