LibJS: Support o.f++ :^)

This patch teaches UpdateExpression how to use a Reference. Some other
changes were necessary to keep tests working:
A Reference can now also refer to a local or global variable. This is
not fully aligned with the spec since we don't have a Record concept.

Libraries/LibJS/Tests/update-expression-on-member-expression.js

@@ -0,0 +1,14 @@
+load("test-common.js");
+
+try {
+    var o = {};
+    o.f = 1;
+
+    assert(o.f++ === 1);
+    assert(++o.f === 3);
+    assert(isNaN(++o.missing));
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}