[FEATURE] Add possibility to center toast vertically

[mateki0]

Is your feature request related to a problem? Please describe.
Currently, it is impossible to center toast vertically without explicitly specifying toast height.

Describe the solution you'd like
I would like to have extended position props with value "center" or "center-vertically" which will set the proper style to the Positioner component

Describe alternatives you've considered
I was thinking about exposing toast height, but it's not a good solution because height is known when it's rendered, and it's being positioned after which looks like a bug.

Additional context
I can make a PR with it if you accept this feature.