LibJS: Implement the Temporal.PlainDateTime constructor

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

Libraries/LibJS/Tests/builtins/Temporal/PlainDateTime/PlainDateTime.js

@@ -0,0 +1,53 @@
+describe("errors", () => {
+    test("called without new", () => {
+        expect(() => {
+            Temporal.PlainDateTime();
+        }).toThrowWithMessage(
+            TypeError,
+            "Temporal.PlainDateTime constructor must be called with 'new'"
+        );
+    });
+
+    test("cannot pass Infinity", () => {
+        for (let i = 0; i < 9; ++i) {
+            const args = Array(9).fill(0);
+
+            args[i] = Infinity;
+            expect(() => {
+                new Temporal.PlainDateTime(...args);
+            }).toThrowWithMessage(RangeError, "Invalid plain date time");
+
+            args[i] = -Infinity;
+            expect(() => {
+                new Temporal.PlainDateTime(...args);
+            }).toThrowWithMessage(RangeError, "Invalid plain date time");
+        }
+    });
+
+    test("cannot pass invalid ISO date or time", () => {
+        // NOTE: The year max value is 3 more than in the polyfill, but they incorrectly seem to
+        // use ISOYearMonthWithinLimits, which AFAICT isn't used for PlainDate{,Time} in the spec.
+        // ¯\_(ツ)_/¯
+        const badValues = [275764, 0, 0, 24, 60, 60, 1000, 1000, 1000];
+        for (let i = 0; i < 9; ++i) {
+            const args = [0, 1, 1, 0, 0, 0, 0, 0, 0];
+            args[i] = badValues[i];
+            expect(() => {
+                new Temporal.PlainDateTime(...args);
+            }).toThrowWithMessage(RangeError, "Invalid plain date time");
+        }
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 3", () => {
+        expect(Temporal.PlainDateTime).toHaveLength(3);
+    });
+
+    test("basic functionality", () => {
+        const plainDateTime = new Temporal.PlainDateTime(2021, 7, 22, 19, 46, 32, 123, 456, 789);
+        expect(typeof plainDateTime).toBe("object");
+        expect(plainDateTime).toBeInstanceOf(Temporal.PlainDateTime);
+        expect(Object.getPrototypeOf(plainDateTime)).toBe(Temporal.PlainDateTime.prototype);
+    });
+});