Rework show for alerts

[imageck]

Is your feature request related to a problem? Please describe

Currently, if it is false, then the component is not rendered. This behavior fails the test for Technique ARIA19.

Describe the solution you'd like

When the page is loaded, the alert container should be present in the DOM but empty and hidden.

Describe alternatives you've considered

Instead, the show prop could be used to determine whether to apply display: none or not. It could also be deprecated in favor of the native hidden attribute, thus leaving it to authors to implement. Another technique is to tie its visibility to the presence of the content inside, hiding it when it's empty. The author would only need to update the alert's content, its visibility would be handled automatically.

Additional context

https://www.w3.org/WAI/ARIA/apg/patterns/alert/examples/alert/
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/alert_role
https://www.w3.org/TR/selectors-4/#the-empty-pseudo