added strings

src/parser.y

@@ -75,6 +75,7 @@ stmt:
 expr:
       NUMBER              { $$ = std::make_unique<Number>($1); }
     | IDENTIFIER          { $$ = std::make_unique<Identifier>($1); }
+    | STRING              { $$ = std::make_unique<String>($1); }
     | expr PLUS expr      { $$ = std::make_unique<BinOp>(
                                 BinaryOperator(BinaryOperator::ADD),
                                 std::move($1), std::move($3)); }