`bevy_input_focus`: Debug navigation visualization

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

For debugging, it would be nice to have some options to display the current input focus navigation graph.

## What solution would you like?

Draw arrows (double headed for symmetric edges) between nodes to show the edges of the navigation graph.

Could have two modes, one that displays the entire graph, and one that only shows the edges from the current input focus.

Start with the bevy_input_focus entry point and trace the navigation graph and current input focus requirements, then inspect existing debug-drawing facilities. Done means the graph can be visualized in whole-graph and current-focus-only modes, with arrows between nodes and double-headed arrows for symmetric edges.

Written by the indexing model from the issue text.