Upgrade success Alerts to role="alert" so they're read consistently

Changes proposed in this pull request:

• Upgrade success Alerts to role="alert" so they're read consistently
• Make aria-live region for users list visible only to screen readers
• Include a prototype page (/prototype/alert) that demonstrates how Alert components are read when they're added dynamically to a page.

See comments on #531 (closed) for further explanation.

Related issues

Part of research for #531 (closed)

Submitter checklist

• Added logging is not capturing sensitive data and is set to an appropriate level (DEBUG vs INFO etc)
• Updated relevant documentation (README, ADRs, explainers, diagrams)

Security considerations

None - UI only