This is a generalization of the existing concept of tri-layer support
that's already well known. Essentially, a conditional-layer
configuration activates a particular layer (the then-layer) when one or
more other layers (the if-layers) are activated.

This is commonly used on ortho keyboards to activate a third "adjust"
layer while the primary two layers ("lower" and "raise") are active.

Implements new hold/tap flavor, tap-unless-interrupted
Adds tests
Adds docs

* 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. 

* Basic driver, using the GD7965 driver as a basis, since the ICs
  are very similar.

Co-authored-by: Pete Johanson <peter@peterjohanson.com>

tapping_term_ms -> tapping-term-ms
quick_tap_ms -> quick-tap-ms

The old style works, although it's deprecated.

feat(combos): add layer filtering

Co-authored-by: KemoNine <mcrosson@kemonine.info>

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.

when quick-release is enabled, the sticky key is released immediately after another key is pressed