LibJS: Allow function calls with missing arguments

We were interpreting "undefined" as a variable lookup failure in some
cases and throwing a ReferenceError exception instead of treating it
as the valid value "undefined".

This patch wraps the result of variable lookup in Optional<>, which
allows us to only throw ReferenceError when lookup actually fails.

Libraries/LibJS/Tests/variable-undefined.js

@@ -0,0 +1,19 @@
+function assert(x) { if (!x) throw 1; }
+
+function foo(a) {
+    return a;
+}
+
+try {
+    var x = undefined;
+    assert(x === undefined);
+    assert(foo(x) === undefined);
+
+    var o = {};
+    o.x = x;
+    assert(o.x === undefined);
+    assert(o.x === x);
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}