[control-state-mixin] Rename focus-ring to focus-visible

[abdonrd]

I know it would be a breaking change, but...

Would it make sense to rename focus-ring to focus-visible? To be closer to the specification: https://developer.mozilla.org/en-US/docs/Web/CSS/:focus-visible

https://github.com/vaadin/component-mixins/blob/c83bb8903ef1a84e011035b3ba30a99dfbc98e7f/packages/focus-visible-mixin/focus-visible-mixin.ts#L76-L82

[web-padawan]

This was considered when working on vaadin/component-mixins#11 and is planned to be done in future.

The only reason to keep it would be consistency, as most of components are still Polymer based (and it's unlikely that we manage to finish the conversion any time soon).