[accessibility] Use `<button>` or `<a role="button">`

[Malvoz]

Time controls are announced as a "link" using a screen reader (missing role="button").

Another option is to use <button type="button">, but which would require additional styling to reset browser defaults.

References:

• https://github.com/Leaflet/Leaflet/blob/bdd7ebf836201550f6d173943b728098b83f40d5/src/control/Control.Zoom.js#L89-L91
• https://www.w3.org/TR/2016/WD-wai-aria-practices-1.1-20160317/examples/button/button.html
• Leaflet Guideline: Test your maps
• Leaflet Plugin Authoring Guide: Content Accessibility