Popover API and top-layer support

**Do you want to request a *feature* or report a *bug*?**
Feature request

**What is the current behavior?**
Toasts appear over the top of the base layer

**What is the expected behavior?**
Toasts should be elevated to the top layer where possible

With the new [Popover API](https://developer.mozilla.org/en-US/docs/Web/API/Popover_API) on the horizon it might be worth looking into allowing toasts to be placed in the top-layer. This will prevent toasts from being behind `dialog`s and `popover` elements which, I imagine, will become a problem in the future.

**Related issues**
[Feature Request: Top Layer Stack Management](https://github.com/whatwg/html/issues/10370)
[API to get top layer elements](https://github.com/whatwg/html/issues/9075)
[A popover on top of a modal dialog should be interactable](https://github.com/whatwg/html/issues/9936)