Support `LAST_VALUE()` and `LEAD()` window functions

[knutwannheden]

Dolt now already has very good support for window functions. Of the remaining few standard window functions I think the LAST_VALUE() and LEAD() functions would be very valuable to also have support for.

[fulghum]

Thanks for all the testing you've been doing for Dolt window functions! You've found some really good issues for us. last_value() and lead() will be awesome to support.

[knutwannheden]

Well, kudos to you all for your responsiveness, successful community building and the fast-paced development of an exciting product!

[timsehn]

I think @jcor11599 could bang these out in an afternoon.

[jycor]

These two window functions have been merged to main, and should be available next release either later this week or early next week.
Let us know how they work for you!