`bevy_input_focus`: Support for navigation in arbitrary directions

## What problem does this solve or what need does it fill?

At the moment directional navigation only supports navigation in eight directions using the `CompassOctant` enum.
But games often have have skill trees and radial menus navigable with analogue sticks in more than eight directions.
Hexagonal maps are also common in games and five directional navigation doesn't map well onto `CompassOctant`.

## What solution would you like?

The neighbours map should support any number of destinations, perhaps by holding a map from a set of disjoint direction ranges to neighbours. Then given a `Dir2` it would find the containing range and map it to a destination.

The API could still allow use of `CompassOctant` but it would be mapped internally to `Dir2`s.