Bug: negative and minus sign lexical conflict

When you write code like the following
```js
const expr = "2 -3";
console.log(evaluate(expr)); // 2
```
You will get `2` instead of `-1`, this is because the tokenizer parses `-3` into a **number** instead of **an operator and a number**. 

If you want to get `-1`,please add a space before `3`, like below
```js
const expr = "2 - 3";
console.log(evaluate(expr)); // -1
```