Support the BigInt global

Using the rather new addition `BigInt`, XO complains about it being undefined. The feature has been at stage 4 since late September 2019 and many browsers and node versions support it.

Example:

```JavaScript
const n = BigInt(5);
```

```
✖   1:9    BigInt is not defined.             no-undef
```

The `BigInt` literal syntax (`5n`) works, however.