Add next-source-p and previous-source-p.

[hgluka]

Description

This PR adds next-source-p and previous-source-p as exported functions.
I did this by making a next-source-position function that next-source-p and next-source can have in common.

Discussion

The implementation might be inefficient, because of the repeated (remove-if #'empty-source-p (sources prompter)) calls, but it seemed like the most elegant way to do it.

CC @aadcg

[aadcg]

@hgluka I didn't look deep into it, but if next-source-position handles non-empty sources then next-source shouldn't need to know about it.

Don't forget to enrich the test suite with these new added functions.

[aadcg]

@hgluka I still think that this solution is subpar. I think everything would become easier if next-source-position returned a source object instead of an index. Then next/previous-source-p would only check whether (next-source-position prompter {1,-1} source) returns non-nil.

[hgluka]

Ah, good point, I didn't think of that. Thanks for the help @aadcg!

[aadcg]

I still think that your solution is subpar. I've pushed some commits.

Does it look good? Am I missing something?

Note that the CI is failing but it's unrelated to this PR.

[hgluka]

@aadcg Thanks, it looks a lot better!
The only nitpick I have is the name adjacent-source, since it only really makes sense for the case when (= steps 1).
Still, that's not a big deal so I think this can be merged.

[hgluka]

@aadcg I just realized that we need next-source-p and previous-source-p to have a source argument.
Slipped my mind when I looked at it earlier, but in Nyxt (in atlas-engineer/nyxt#3202, at least) we want to use these functions on all the sources in a prompt buffer, not just the current-source.

[aadcg]

@hgluka you can still use adjacent-source no?

[hgluka]

Oh, adjacent-source is exported as well, so yes I can.