Handle notifications

Not sure how we'd handle this but would be nice to show the user `notifications` coming across the wire.

for metamask `accountChanged` for example is a notification that just comes across the wire.

also `eth_subscribe` kicks off notifications that come back with a notificationId via `eth_subscription` notification.

We need to handle these and show them to the user somehow.

The issue names no files, tests, or entry points. Start by locating how the interactive documentation receives and displays messages, then investigate the accountChanged and eth_subscription notification flows. Done should include a defined user-facing treatment for both unsolicited notifications and subscription notifications.

Written by the indexing model from the issue text.