Add support for BigInt

[vladfrangu]

Fixes #39

• Implements support for the BigInt, BigInt64Array and BigUint64Array

• All tests pass

[sindresorhus]

Why are you removing Node.js 8 support? That seems unrelated.

[vladfrangu]

I thought nodejs 8 didn't have bigint support. Let me check real quick...

[vladfrangu]

nodejs 8 does NOT have BigInt support. (quite a shame too)

EDIT: There probably is bigint support in nodejs 8 through a harmony flag, but that's still far more than the average person will do...

[sindresorhus]

We're not actually using BigInt, we're just detecting it, so we can still preserve support for Node.js 8. You can just comment out the tests for now.

[vladfrangu]

Yep, after reading your review, I realized you only did typeof / toString calls and checks, which would mean nodejs 8 support would still be present! I've done your requested changes now, as well as commented out all tests that only work on nodejs >=10. Hope this helps!

[vladfrangu]

@sindresorhus should be done now! If there's any other changes you'd like me to do, please let me know 👍

[sindresorhus]

Nice work 👌