LibJS: Fix impossible member access for negative integers

The PropertyName class able to match a number or an array can only
accept positive numerical values. However, the computed_property_name
method sometimes returned negative values.

This commit also adds a basic object access test case.

Libraries/LibJS/Tests/object-basic.js

@@ -0,0 +1,17 @@
+try {
+    var o = { foo: "bar" };
+    assert(o.foo === "bar");
+    assert(o["foo"] === "bar");
+    o.baz = "test";
+    assert(o.baz === "test");
+    assert(o["baz"] === "test");
+    o[10] = "123";
+    assert(o[10] === "123");
+    assert(o["10"] === "123");
+    o[-1] = "hello friends";
+    assert(o[-1] === "hello friends");
+    assert(o["-1"] === "hello friends");
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}