Add support for a "keepMounted"-property on Tabs

[dmerckx]

Currently, tabs children are unmounted when you navigate inbetween them.

Sometimes this is desirable, but often I want my form elements to remain mounted to keep their state. An option like this would make that easier to implement.

Would be nice to have some support for managing browser form validation for hidden tabs too (not sure how that would be implemented best).

[benmerckx]

Something for https://react.dev/reference/react/Activity?