Improve discoverability of chat participants

[charris-msft]

@roblourens & @joyceerhl
In the "VS Code UX Sync" meeting on 2024.08.20, we discussed ways to improve the discoverability of chat participants. This is important so we can help users become aware of participants that could be very helpful for them.

Here are the top suggestions from the meeting:

• Recommend strategic participants that are not installed
  • Copilot Chat has metadata for a select set of participants, even when those participants are not installed.
• Include strategic participants in the @ menu, even if they are not installed
  • when the user types @, they would see participants that are not yet installed.
  • when a participant that is not installed is used, the extension will be installed

Examples of strategic participants:

• @Azure
• ...

[joyceerhl]

Include strategic participants in the @ menu, even if they are not installed

This makes sense to me, we do something similar in the remote indicator picker (bottom left corner) for remote extensions that are not yet installed.