Support the BigInt global

[AlexGustafsson]

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:

const n = BigInt(5);

✖   1:9    BigInt is not defined.             no-undef

The BigInt literal syntax (5n) works, however.