UI: Add Notice.List component that with live announcements behavior #84941

What problem does this address?

With #82701, the speak behavior was removed from the Notice component, with the proposal in #82701 (comment) being that the spoken announcements should be handled by a Notice.List component.

The problem this solves is that while it doesn't make sense that the notice UI primitive handles spoken announcements, there's still a need for an abstraction that helps ensure correct behaviors around announcing dynamic notice content.

What is your proposed solution?

A Notice.List is a suggested abstraction that handles both visual layout as well as spoken announcement behaviors in response to adding and removing notices dynamically from the list.

Via #82701 (comment) :

• Notice.List renders its own live region container whose content is populated by context-registered title and description of descendent notices, something of a hybrid between BaseUI's Toast.Viewport and UI: Update Notice spoken message default to plaintext title and description #80706
• The actual live region might be children of the container element itself, alongside the rendered notices. There would be a live region per politeness, and we infer politeness level from intent (maybe configurable).
• The live regions aren't rendered if the list has focus, to address @joedolson 's point in UI: Remove speak functionality from Notice primitive #82701 (comment), and following BaseUI's similar implementation