LibJS: Add a scope object abstraction

Both GlobalObject and LexicalEnvironment now inherit from ScopeObject,
and the VM's call frames point to a ScopeObject chain rather than just
a LexicalEnvironment chain.

This gives us much more flexibility to implement things like "with",
and also unifies some of the code paths that previously required
special handling of the global object.

There's a bunch of more cleanup that can be done in the wake of this
change, and there might be some oversights in the handling of the
"super" keyword, but this generally seems like a good architectural
improvement. :^)

Libraries/LibJS/Interpreter.h

@@ -71,7 +71,8 @@ public:
     Heap& heap() { return vm().heap(); }
     Exception* exception() { return vm().exception(); }
 
-    LexicalEnvironment* current_environment() { return vm().current_environment(); }
+    ScopeObject* current_scope() { return vm().current_scope(); }
+    LexicalEnvironment* current_environment();
 
     void enter_scope(const ScopeNode&, ArgumentVector, ScopeType, GlobalObject&);
     void exit_scope(const ScopeNode&);