Make the disabled state of zoom controls available to ATs

The leaflet-disabled class is set on zoom controls: https://github.com/Leaflet/Leaflet/blob/71f73785c91f50821cee4f590c41b14eeddf394c/src/control/Control.Zoom.js#L102-L104 and styles are applied to convey a “disabled” state: https://github.com/Leaflet/Leaflet/blob/71f73785c91f50821cee4f590c41b14eeddf394c/dist/leaflet.css#L310-L314

the zoom controls remain operable however, and the state is not available to assistive technologies such as screen readers. There are at least 3 ways to convey the disabled state of buttons:

• tabindex="-1" - Bing Maps for example, uses this approach. Though it may be confusing to keyboard-only users, as the button is then removed from the tab order.
• The disabled attribute - Not applicable (Leaflet uses <a role="button"> as opposed to <button>).
• aria-disabled="true" - May or may not be as well supported as a negative tabindex or the disabled attribute, it does seem to be well supported though.

Whether tabindex="-1" or aria-disabled="true|false" is used (I personally vote for the latter), it could be set in addition to the leaflet-disabled class for backwards compatibility.

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Implementing this may satisfy (parts of) the WCAG 2.1 Success Criterion: 1.3.1 Info and Relationships and 4.1.2 Name, Role, Value for the zoom controls.

[IvanSanchez]

@AkshataJ96 Nobody seems to be working on this on the moment, feel free to pick it up.

[Malvoz]

Fixed in https://github.com/Leaflet/Leaflet/pull/7280, thanks!