LibJS: Add the WeakRef.prototype.deref method

This is the only exposed method of the WeakRef built-in.

Userland/Libraries/LibJS/Tests/builtins/WeakRef/WeakRef.prototype.deref.js

@@ -0,0 +1,21 @@
+test("length is 0", () => {
+    expect(WeakRef.prototype.deref).toHaveLength(0);
+});
+
+test("basic functionality", () => {
+    var original = { a: 1 };
+    var weakRef = new WeakRef(original);
+
+    expect(weakRef.deref()).toBe(original);
+});
+
+test("kept alive for current synchronous execution sequence", () => {
+    var weakRef;
+    {
+        weakRef = new WeakRef({ a: 1 });
+    }
+    weakRef.deref();
+    gc();
+    // This is fine 🔥
+    expect(weakRef.deref()).not.toBe(undefined);
+});