LibJS: Perform function instantiation in bytecode

This replaces Bytecode::Op::EnterScope with a new NewFunction op that
instantiates a ScriptFunction from a given FunctionNode (AST).

This is then used to instantiate the local functions directly from
bytecode when entering a ScopeNode. :^)

Userland/Libraries/LibJS/Forward.h

@@ -114,6 +114,7 @@ class Error;
 class ErrorType;
 class Exception;
 class Expression;
+class FunctionNode;
 class Accessor;
 class GlobalObject;
 class HandleImpl;