[accessibility] Keyboard-navigable radio buttons in `geojson-list-group`

[Malvoz]

The radio buttons in the radio group are currently not keyboard-navigable using the arrow keys.

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Other resources that may be helpful:

• Leaflet guide to accessibility: Test your maps
• Leaflet Plugin Authoring Guide: Content Accessibility

[stefanocudini]

please send a PR for this

[stefanocudini]

now is work in progress here:
https://github.com/stefanocudini/leaflet-geojson-selector/tree/handle-keyboard-selection

@Malvoz if you want contribute you are welcome