LibJS: Start implementing Temporal.Calendar

Just like the previous Temporal.{Instant,TimeZone} commits, this patch
adds the Calendar object itself, its constructor and prototype
(currently empty), and two required abstract operations.

Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.js

@@ -0,0 +1,38 @@
+describe("errors", () => {
+    test("called without new", () => {
+        expect(() => {
+            Temporal.Calendar();
+        }).toThrowWithMessage(TypeError, "Temporal.Calendar constructor must be called with 'new'");
+    });
+
+    test("argument must be coercible to string", () => {
+        expect(() => {
+            new Temporal.Calendar({
+                toString() {
+                    throw new Error();
+                },
+            });
+        }).toThrow(Error);
+    });
+
+    test("invalid calendar identifier", () => {
+        expect(() => {
+            new Temporal.Calendar("foo");
+        }).toThrowWithMessage(RangeError, "Invalid calendar identifier 'foo'");
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.Calendar).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const calendar = new Temporal.Calendar("iso8601");
+        // FIXME: Enable this once Temporal.Calendar.prototype.id is implemented
+        // expect(calendar.id).toBe("iso8601");
+        expect(typeof calendar).toBe("object");
+        expect(calendar).toBeInstanceOf(Temporal.Calendar);
+        expect(Object.getPrototypeOf(calendar)).toBe(Temporal.Calendar.prototype);
+    });
+});