Add content element for Toasts

It would be great to have a content element to create Toasts!
It should then be possible to add various content elements into the Toast, like tabs, carousels etc.

Start by locating how existing content elements and Toasts are implemented in the repository, then review how nested elements such as tabs and carousels are supported. Done means a Toast content element can contain varied content elements and renders them correctly, with the required behavior covered by the project's existing checks.

Written by the indexing model from the issue text.