feat: Add indices_threshold function

[Pringled]

This PR adds an indices_threshold function that returns unsorted indices above a threshold, a faster alternative to nearest_neighbors_threshold if you don't need sorted results.

[stephantul]

Some small comments. I see two paths forward:

1. also make a most_similar version
2. collapse the helper function into the body of the main function you added.

I'm a bigger fan of 2). I don't think anyone would use it.