feat(api): add name argument to topk/value_counts

[jcrist]

This adds a new name argument to topk & value_counts. Fixes #9948.

I think adding a name arg to these helpers makes sense. For other builtin aggregate methods you can chain a .name("myname") call on the end, but in these cases the return type is a Table. Adding an option to specify the name of the new column is nice for ux.

I opted to go for a simple name: str | None version, rather than supporting format strings/callables like rename. If a user wants to programmatically generate the name then they can do so themselves outside of this method. Keep it simple.

One open question is whether this argument should be keyword only. I went with positional or keyword for now, no strong thoughts.

Also added a docstring example for topk while I was at it.

[cpcloud]

Slight preference for keyword only, since both by and name can be strings, so without naming anything things can get slightly hard to read.

[NickCrews]

This looks perfect, thanks @jcrist !