types: define BigInt

[eush77]

This PR defines BigInt type.

Since it requires ES2020, update ESLint configuration for testing. This triggered comma-dangle in a bunch of places for me, so I fixed those, too.

The actual functional change in index.js does not require ES2020.

[davidchambers]

Thank you very much for this contribution, @eush77. I'm sorry for taking so long to review it!

I would like to reduce the scope of the changes. If we remove 1n from the test suite, setting env.es2020 to true should suffice. We need not test the literal form given that BigInt (1) evaluates to the same value.

What do you think? I realize that months have passed and you've no doubt moved on to other things. If you'd like to update the pull request yourself, I promise not to leave you hanging again. If you'd prefer for me to make the changes, just say so.

Thanks again for making Sanctuary better. ❤️