- Add custom handlers on top of the standard modes as well. Previously this was only possible for mode == "custom".
  - The goal here is to make it possible to move all input handling inside `setUI` where today some apps add on extra handlers outside of `setUI` calls.
- Change the default behaviour of the hardware button to act immediately on press down. Previously it has been acting on button release.
  - This makes the interaction slightly snappier.
  - In addition to the existing `btn` key a new `btnRelease` key can now be specified. `btnRelease` will let you listen to the rising edge of the hardware button.

Explicitly state button should fire on falling edge.

Does not change `btn` mapped handlers to act on rising edge just jet.

As per suggestion in
espruino#2571 (comment).