#7884599 Site Assembler – Snackbar notifications as in the editor

Related to #74758

Proposed Changes

• Add feature flag pattern-assembler/notices
• Use withNotices HOC to handle the notices state
• Use SnackbarList component to handle the stack of notices.
• Show a notice stacked (as in the editor but on the bottom-right of the screen) when any pattern is added, replaced, or removed.
• Use the same notice copy as the editor Block pattern "Name" inserted.
• Auto dismisses after 10 seconds (as in the editor). The time is hardcoded in the component, maybe for accessibility reasons.
• Dismiss a notice immediately on click (as in the editor).

Testing Instructions

• Click on the Calypso Live link from the first comment
• Create a new site
• Continue until the design picker
• Scroll down and click Start designing
• Click Homepage > Add patterns to add patterns and verify that the notice shows the category name of the patterns added
• Replace the patterns and verify that the notice shows the category name of the new pattern and says replaced.
• Remove patterns to verify that the notice says removed
• Click on a notice to dismiss it
• Wait 10 seconds to see the notice disappear
• Click the < back or Save button and verify the notices stay
• Go back to the design picker or click Continue to verify the notices disappear

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?