LibJS: Add the Set built-in object

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

@@ -0,0 +1,31 @@
+test("constructor properties", () => {
+    expect(Set).toHaveLength(0);
+    expect(Set.name).toBe("Set");
+});
+
+describe("errors", () => {
+    test("invalid array iterators", () => {
+        [-100, Infinity, NaN, {}, 152n].forEach(value => {
+            expect(() => {
+                new Set(value);
+            }).toThrowWithMessage(TypeError, "is not iterable");
+        });
+    });
+    test("called without new", () => {
+        expect(() => {
+            Set();
+        }).toThrowWithMessage(TypeError, "Set constructor must be called with 'new'");
+    });
+});
+
+describe("normal behavior", () => {
+    test("typeof", () => {
+        expect(typeof new Set()).toBe("object");
+    });
+
+    test("constructor with single array argument", () => {
+        var a = new Set([0, 1, 2]);
+        expect(a instanceof Set).toBeTrue();
+        expect(a).toHaveSize(3);
+    });
+});