LibJS: Implement the Temporal.PlainTime constructor

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

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

@@ -0,0 +1,50 @@
+describe("errors", () => {
+    test("called without new", () => {
+        expect(() => {
+            Temporal.PlainTime();
+        }).toThrowWithMessage(
+            TypeError,
+            "Temporal.PlainTime constructor must be called with 'new'"
+        );
+    });
+
+    test("cannot pass Infinity", () => {
+        for (let i = 0; i < 6; ++i) {
+            const args = Array(6).fill(0);
+
+            args[i] = Infinity;
+            expect(() => {
+                new Temporal.PlainTime(...args);
+            }).toThrowWithMessage(RangeError, "Invalid plain time");
+
+            args[i] = -Infinity;
+            expect(() => {
+                new Temporal.PlainTime(...args);
+            }).toThrowWithMessage(RangeError, "Invalid plain time");
+        }
+    });
+
+    test("cannot pass invalid ISO time", () => {
+        const badValues = [24, 60, 60, 1000, 1000, 1000];
+        for (let i = 0; i < 6; ++i) {
+            const args = [0, 0, 0, 0, 0, 0];
+            args[i] = badValues[i];
+            expect(() => {
+                new Temporal.PlainTime(...args);
+            }).toThrowWithMessage(RangeError, "Invalid plain time");
+        }
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 0", () => {
+        expect(Temporal.PlainTime).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        const plainTime = new Temporal.PlainTime(19, 46, 32, 123, 456, 789);
+        expect(typeof plainTime).toBe("object");
+        expect(plainTime).toBeInstanceOf(Temporal.PlainTime);
+        expect(Object.getPrototypeOf(plainTime)).toBe(Temporal.PlainTime.prototype);
+    });
+});