Feat: Make mobile offsets configurable

[Achder]

I'm currently implementing a mobile app and sonner is a great fit as it supports swiping and all.
There is just one thing that I needed to hack around:

The mobile offsets to the sides are currently hardcoded to 16px for all screens smaller than 600px. See here:

sonner/src/styles.css

Line 354 in a869bd5

--mobile-offset: 16px;

It would be great to make these configurable, especially since all the desktop stuff already supports this.