LibJS: Start implementing ShadowRealm

This commit adds the ShadowRealm object itself, its constructor, and
prototype (currently empty).

Userland/Libraries/LibJS/Tests/builtins/ShadowRealm/ShadowRealm.js

@@ -0,0 +1,20 @@
+describe("errors", () => {
+    test("called without new", () => {
+        expect(() => {
+            ShadowRealm();
+        }).toThrowWithMessage(TypeError, "ShadowRealm constructor must be called with 'new'");
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 0", () => {
+        expect(ShadowRealm).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        const shadowRealm = new ShadowRealm();
+        expect(typeof shadowRealm).toBe("object");
+        expect(shadowRealm).toBeInstanceOf(ShadowRealm);
+        expect(Object.getPrototypeOf(shadowRealm)).toBe(ShadowRealm.prototype);
+    });
+});