feat: add more window functions

[richtia]

PR to add more window functions:

• first_value
• last_value
• nth_value
• lead
• lag

[jacques-n]

mostly looks good but a couple of return types seem weird.

[westonpace]

I have a few requests. Also, I'm not sure we need to say things like "expression is evaluated against". We don't really use that terminology elsewhere. For example, we could phrase addition as:

Two different expressions are evaluated. The results of these expressions are added together.

But I think that is more confusing. In other words, I think:

Returns the first value in the window

is more clear than:

The expression is evaluated against the first row.

[westonpace]

A few more nitpicks

[EpsilonPrime]

Looks like there's one failing check to address before we can proceed here.

[richtia]

Looks like there's one failing check to address before we can proceed here.

Thanks! Had a typo!

[EpsilonPrime]

@westonpace Anything else you'd like to see?

[westonpace]

The return of first_value and last_value is only nullable if the input is nullable. I don't remember if we have syntax for that or not. But this is minor and we can always adjust later if needed.