feat: Add cancelPrompt handlers for DuoChat

Description

This MR adds the handlers and logic for implementing the cancel prompt button in Duo Chat.

Related Issues

Resolves #1332 (closed).

How has this been tested?

To test the feature, checkout this branch and run any required npm installs to ensure that at least @gitlab/ui version 85.12.2 is being used.

Then use GitLab Duo chat as you would normally, but if you send the wrong message, feel free to cancel and send another!

Screen_Recording_2024-08-05_at_6.12.22_PM

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes