Add a Snackbar component to expo-ui

Hey team — I know you're steering the expo-ui effort and the direction so far has been great. But one critical piece is still missing: a proper Snackbar component, comparable to what Jetpack Compose offers.

In almost every production mobile app, transient in-app notifications are essential — for undo actions, connection status, form feedback, or lightweight alerts. A toast is too ephemeral, a modal is too disruptive. Snackbars hit that perfect middle ground, and they're a first-class citizen in Material Design for a reason.

Right now, teams building with Expo have to either pull in a third-party library, hack a custom one with Animated and absolute positioning, or just skip the pattern entirely — all of which undermines the “fully native, fully Expo” experience that expo-ui promises.

I’d love to see a Snackbar component that:

· Anchors to the bottom of the screen and slides up

· Supports an optional action (e.g., “UNDO”) with proper touch handling

· Respects safe areas and keyboard insets out of the box

· Follows the duration/sweep-to-dismiss behaviors we’re used to from Material you/AOSP

This is not a “nice to have” — it's a baseline primitive for modern mobile UX. Can we get this on the roadmap?