Better support for /help with chat agents

[roblourens]

We should either

• Make vscode own /help. It lists all commands and the default agent is able to contribute a paragraph
  • I think I prefer this option. /help should list commands from extensions, but there is no reason for the default agent to be able to get a list of these. But it does still need heavy customization from the default agent to look nice and match our current experience.
• The default agent owns it, and is able to get a list of all agents/commands, including from other extensions
  • Need special API to contribute the slash command and get the list of agents

Forked from https://github.com/microsoft/vscode-copilot/issues/1682

10/31/23 API call notes

• This can be owned by vscode, and probably the extension can just contribute the last section after the list of agents/commands

[roblourens]

11/7/23 call notes

• VS Code could own the top-level /help to list only agents, and create /help per-agent that lists available slash commands.
• Agents can declare a "short" and "detailed" description that shows up in these commands.

[roblourens]

The current plan

• /help only shows agents, not slash commands
• @agent /help is auto-generated for each agent, and shows slash commands. The agent can contribute some text to customize it.
• If I run /help and click the @agent, it runs @agent /help