LibJS: Add empty bytecode generation for VariableDeclaration

These will be partly handled by the relevant ScopeNode due to
hoisting, same basic idea as function declarations.

VariableDeclaration needs to do some work, but let's stub it out
first and start empty.

Userland/Libraries/LibJS/AST.h

@@ -997,6 +997,7 @@ public:
 
     virtual Value execute(Interpreter&, GlobalObject&) const override;
     virtual void dump(int indent) const override;
+    virtual void generate_bytecode(Bytecode::Generator&) const override;
 
     const NonnullRefPtrVector<VariableDeclarator>& declarations() const { return m_declarations; }
 

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -556,6 +556,10 @@ void FunctionDeclaration::generate_bytecode(Bytecode::Generator&) const
 {
 }
 
+void VariableDeclaration::generate_bytecode(Bytecode::Generator&) const
+{
+}
+
 void CallExpression::generate_bytecode(Bytecode::Generator& generator) const
 {
     m_callee->generate_bytecode(generator);