feat: MultiCount Interface

[wegreenall]

Adds a MultiCount interface. This extends the MultiSelect to allow for (integer) numerical counts. Rather than doing anything complicated, the count is incremented/decremented with the right and left arrows, and incremented/decremented by ten using shift and right, left arrow respectively. I stuck close to the MultiSelect paradigm to minimise the difference from old code in large commits for new features, to make it easier to compare.

If accepted I will add an appropriate README paragraph (it felt rather brash otherwise).

[mikaelmello]

Hey! This looks pretty great impl-wise, I'm interested in hearing more about the use-cases for this prompt type.

Was there something specific you needed that made you create this PR?

[wegreenall]

Hi! Thanks for getting back to me.

Yes, I had a situation where single units (As in MultiSelect) were not enough (specifically, billable hours in different categories).
I originally wanted to have a way to type in the number but I think that perhaps what I have done here is a good middle ground between your other prompt forms and exactly doing what I had envisioned.

I'll get on with the conversion to a struct and get back to you.

[wegreenall]

One question:
I note that in multiselect you pull the checkbox off the render_config struct (you use it in MultiSelectBackend). I did not see an simple way, Other than adding a field to said struct, to store the equivalent "countbox". I therefore define it directly inside
render_options on impl MultiCountBackend - see backend.rs line 492. However I don't think this is the right way to do it, but couldn't tell the best way that fits in with the rest of the code. What do you suggest? I appreciate there is a lot of stuff to look through.

[wegreenall]

Would you be interested in this feature being included in repository? If so, is there anything else you need from me?

[mikaelmello]

Hey @wegreenall, I think this would be a nice addition yes, just haven't had the proper time to go through the PR yet