Dark toast variant

[vanillajonathan]

I would like to propose the addition of a dark variant of the toast component.

[KrisKodira]

Maybe something like this:

[bardiharborow]

Will likely be shipped with #27514.

[vanillajonathan]

But it would be nice to be able to use the dark toast variant on light backgrounds too. Because if you do it without the header, and position it at the bottom of the screen then it looks like the snackbar component from Material Design as used in Android.