[toast] Add stack mode

[segunadebayo]

🚀 Feature request

Based on https://vercel.com/design/toast, we could design the toast machine to accept the stack option. In this mode, the toast stack on top of themselves by default (with the newest on top), and on hover, they expand correctly.

[ricardo-valero]

Might be worth taking a look at https://github.com/emilkowalski/sonner