* Add optional `hold-trigger-key-positions` hold-tap configuration
* Leverage configuration for decision making around when to trigger
hold decision in hold-taps.
* Add docs for new configuration.
* Tests for the new config/decision logic.
Tap-and-hold a hold-tap to hold the tap behavior so it can repeat.
After a tap, if the same key is pressed within `quick_tap_ms`, the
tap behavior is always picked.
This is useful for things like `&ht LSHFT BACKSPACE` where holding
the backspace is required.
Implements #288.
jding