[accessibility] Fullscreen control should have `role="button"`

[Malvoz]

For consistency with the other controls:
https://github.com/Leaflet/Leaflet/blob/bdd7ebf836201550f6d173943b728098b83f40d5/src/control/Control.Zoom.js#L89-L91

[Malvoz]

Other potentially helpful resources:

• Leaflet Guideline: Test your maps
• Leaflet Plugin Authoring Guide: Content Accessibility