LibJS: Add object literal getter/setter shorthand

Adds support for the following syntax:

let foo = {
    get x() {
        // ...
    },
    set x(value) {
        // ...
    }
}

Libraries/LibJS/Tests/object-getter-setter-shorthand.js

@@ -0,0 +1,50 @@
+load("test-common.js")
+
+try {
+    let o = {
+        get() { return 5; },
+        set() { return 10; },
+    };
+    assert(o.get() === 5);
+    assert(o.set() === 10);
+
+    o = {
+        get x() { return 5; },
+        set x(_) { },
+    };
+    assert(o.x === 5);
+    o.x = 10;
+    assert(o.x === 5);
+
+    o = {
+        get x() {
+            return this._x + 1;
+        },
+        set x(value) {
+            this._x = value + 1;
+        },
+    };
+
+    assert(isNaN(o.x));
+    o.x = 10;
+    assert(o.x === 12);
+    o.x = 20;
+    assert(o.x === 22);
+
+    o = {
+        get x() { return 5; },
+        get x() { return 10; },
+    };
+
+    assert(o.x === 10);
+
+    o = {
+        set x(value) { return 10; },
+    };
+
+    assert((o.x = 20) === 20);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}