Support multiple, isolated LiveSockets on one page

[mat-hek]

This is an updated and extended version of #3564. Beyond viewSelector option, it also provides better separation between LiveSockets, so that one LiveSocket's root can be embedded in DOM of a view rendered by another one.

This addresses the main limitation preventing Local LiveView from relying solely on LV's public API.

If this gets merged, I'd like to contribute a public way to destroy LiveSockets, also needed by Local LiveView 😉

[mat-hek]

I figured that introducing viewSelector is problematic in many ways. The biggest problem is to make sure that roots belonging to a scoped socket aren't joined by any other socket, and they're there while their socket connects. Another issue is handling navigation and sticky views properly.

Thus, I came up with a different API: LiveSocket.embed(container, url), that makes LV keep more control over when the socket is connected, how the roots are rendered and joined. The main assumptions are that embedded sockets:

• either leave outside of the main's root or in a phx-ignored container
• don't trigger/handle navigation
• can render sticky views, but they're no different than regular views (they're sticky with respect to their socket, and that socket never navigates)

The implementation is partially vibe coded and I'm going to review it in detail, but meanwhile I'm curious what you think.