Request for `NTILE` function

[samjewell]

Hey team. Thanks again for Dolt ❤

I saw:

If you need one that isn't implemented, please file an issue. We can fulfill most requests for new functions within 24 hours.

For me, NTILE is a big one - I'd love for you to add this, to support bucketing and percentile-based groupings. Thanks

[timsehn]

@jycor will work on this today.