LibJS: Implement Temporal.PlainDate.prototype.with()

With one caveat: in the PreparePartialTemporalFields AO I made a change
to fix a spec issue that would require the input object to always have a
month or monthCode property.
This is tracked in https://github.com/tc39/proposal-temporal/issues/1910
and may get accepted as-is, in which case we simply need to remove the
NOTE comment.

Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDate/PlainDate.prototype.with.js

@@ -0,0 +1,58 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.PlainDate.prototype.with).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const plainDate = new Temporal.PlainDate(1970, 1, 1);
+        const values = [
+            [{ year: 2021 }, new Temporal.PlainDate(2021, 1, 1)],
+            [{ year: 2021, month: 7 }, new Temporal.PlainDate(2021, 7, 1)],
+            [{ year: 2021, month: 7, day: 6 }, new Temporal.PlainDate(2021, 7, 6)],
+            [{ year: 2021, monthCode: "M07", day: 6 }, new Temporal.PlainDate(2021, 7, 6)],
+        ];
+        for (const [arg, expected] of values) {
+            expect(plainDate.with(arg).equals(expected)).toBeTrue();
+        }
+
+        // Supplying the same values doesn't change the date, but still creates a new object
+        const plainDateLike = { year: plainDate.year, month: plainDate.month, day: plainDate.day };
+        expect(plainDate.with(plainDateLike)).not.toBe(plainDate);
+        expect(plainDate.with(plainDateLike).equals(plainDate)).toBeTrue();
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.PlainDate object", () => {
+        expect(() => {
+            Temporal.PlainDate.prototype.with.call("foo");
+        }).toThrowWithMessage(TypeError, "Not an object of type Temporal.PlainDate");
+    });
+
+    test("argument must be an object", () => {
+        expect(() => {
+            new Temporal.PlainDate(1970, 1, 1).with("foo");
+        }).toThrowWithMessage(TypeError, "foo is not an object");
+        expect(() => {
+            new Temporal.PlainDate(1970, 1, 1).with(42);
+        }).toThrowWithMessage(TypeError, "42 is not an object");
+    });
+
+    test("argument must have one of 'day', 'month', 'monthCode', 'year'", () => {
+        expect(() => {
+            new Temporal.PlainDate(1970, 1, 1).with({});
+        }).toThrowWithMessage(
+            TypeError,
+            "Object must have at least one of the following properties: day, month, monthCode, year"
+        );
+    });
+
+    test("argument must not have 'calendar' or 'timeZone'", () => {
+        expect(() => {
+            new Temporal.PlainDate(1970, 1, 1).with({ calendar: {} });
+        }).toThrowWithMessage(TypeError, "Object must not have a defined calendar property");
+        expect(() => {
+            new Temporal.PlainDate(1970, 1, 1).with({ timeZone: {} });
+        }).toThrowWithMessage(TypeError, "Object must not have a defined timeZone property");
+    });
+});