Studio: Add UI for clearing chat messages

[katinthehatsite]

Closes https://github.com/Automattic/dotcom-forge/issues/7697
Related to https://github.com/Automattic/dotcom-forge/issues/7411

Proposed Changes

This PR adds a dropdown to clear chat messages.

Testing Instructions

• Pull the changes from this branch
• Start the app with the AI assistant STUDIO_AI=true npm start
• Navigate to the assistant tab and start conversation with the AI assistant
• Once there are a couple of messages, click on the three dots by the input field
• Observe the dropdown to Clear chat history
• Observe the popup that asks for confirmation to delete the chat (not sure if this is completely necessary to have?)
• Confirm the deletion of the chat history
• Observe that the messages are removed and you are back to seeing the welcome messages and prompts

The was already the PR that attempted this change before so I reused some of the code from https://github.com/Automattic/studio/pull/206/files

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[jeroenpf]

Nice work 👍 LGTM!