Add some utilities for the keymap rebinding

[quasilyte]

To make the game really polished, a keymap rebinding can be a desirable feature.

I would like to have some helpers provided by this library.

[quasilyte]

The first step is completed: https://github.com/quasilyte/ebitengine-input/pull/28

I'll test it in my game before making any more progress.