Closes #33637
Closes #37332
and solves part of
https://github.com/zed-industries/zed/discussions/33580#discussioncomment-14195506

This improves the "C" and "alt-C" actions to work like helix.
It also adds "," which removes all but the newest cursors. In helix the
one that's left would be the primary selection, but I don't think that
has an equivalent yet, so this simulates what would be the primary
selection if it was never cycled with "(" ")".

Release Notes:

- Improved multicursor creation and deletion in helix mode

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Co-authored-by: Jakub Konka <kubkon@jakubkonka.com>