Provide warning when a switch has a mixed value (which is not to spec)

Provide at least a warning when an element with a `switch` role has a `mixed` value, which is against specification.

> The [aria-checked](https://www.w3.org/TR/wai-aria-1.1/#aria-checked) [attribute](https://www.w3.org/TR/wai-aria-1.1/#dfn-attribute) of a switch indicates whether the input is on (true) or off (false). The mixed value is invalid, and user agents MUST treat a mixed value as equivalent to false for this role.

References:
https://www.w3.org/TR/wai-aria-1.1/#switch
https://adrianroselli.com/2021/10/switch-role-support.html#Testing