feat(behaviors): Custom Retro Tap Behavior

[nickconway]

This PR adds the ability to have a custom binding for a retro-tap, giving 3 possible bindings on a hold-tap.

[voidyourwarranty2]

As my current keymap depends on this patch, I have produced an updated version that works with the current main branch of ZMK (a lot has changed in the hold-tap behavior since): https://github.com/voidyourwarranty2/zmk/tree/devel-retrotap