LibJS: Add a new EnterScope bytecode instruction

This is intended to perform the same duties as enter_scope() does in
the AST tree-walk interpreter:

- Hoisted function declaration processing
- Hoisted variable declaration processing
- ... maybe more

This first cut only implements the function declaration processing.

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -265,4 +265,19 @@ private:
     Optional<Label> m_target;
 };
 
+class EnterScope final : public Instruction {
+public:
+    explicit EnterScope(ScopeNode const& scope_node)
+        : m_scope_node(scope_node)
+    {
+    }
+
+    virtual ~EnterScope() override { }
+    virtual void execute(Bytecode::Interpreter&) const override;
+    virtual String to_string() const override;
+
+private:
+    ScopeNode const& m_scope_node;
+};
+
 }