[accessibility] `<a href="#">` should use `role="button"`

[Malvoz]

To convey that the <a> is a button and not a link (align with https://github.com/Leaflet/Leaflet/blob/bdd7ebf836201550f6d173943b728098b83f40d5/src/control/Control.Zoom.js#L89-L91).

[Malvoz]

Other potentially helpful resources:

• The WAI-ARIA Authoring Practices recommended keyboard interactions for buttons.
• Leaflet Guideline: Test your maps
• Leaflet Plugin Authoring Guide: Content Accessibility