LibJS: Reduce number of proxy traps called during for..in iteration

Before this change, we would enumerate all the keys with
[[OwnPropertyKeys]], and then do [[GetOwnPropertyDescriptor]] twice for
each key as we went through them.

We now only do one [[GetOwnPropertyDescriptor]] per key, which
drastically reduces the number of proxy traps when those are involved.
The new trap sequence matches what you get with V8, so I don't think
anyone will be unpleasantly surprised here.

Libraries/LibJS/Tests/builtins/Proxy/for-in-iteration-traps.js

@@ -0,0 +1,40 @@
+test("for..in iteration Proxy traps", () => {
+    let traps = [];
+    let array = [1, 2, 3];
+    let from = new Proxy(array, {
+        getPrototypeOf: function (t) {
+            traps.push("getPrototypeOf");
+            return Reflect.getPrototypeOf(t);
+        },
+        ownKeys: function (t) {
+            traps.push("ownKeys");
+            return Reflect.ownKeys(t);
+        },
+        has: function (t, p) {
+            traps.push("has");
+            return Reflect.has(t, p);
+        },
+        getOwnPropertyDescriptor: function (t, p) {
+            traps.push("getOwnPropertyDescriptor");
+            return Reflect.getOwnPropertyDescriptor(t, p);
+        },
+    });
+    const to = [];
+    for (const prop in from) {
+        to.push(prop);
+        from.pop();
+    }
+
+    expect(to).toEqual(["0", "1"]);
+
+    expect(traps).toEqual([
+        "ownKeys",
+        "getPrototypeOf",
+        "getOwnPropertyDescriptor",
+        "getOwnPropertyDescriptor",
+        "getOwnPropertyDescriptor",
+        "getOwnPropertyDescriptor",
+        "getOwnPropertyDescriptor",
+        "getOwnPropertyDescriptor",
+    ]);
+});