[Meta] Implement triagebot commands for each step of our procedure

[spastorino]

This is a tracking issue about something that is still il-defined but the idea would be to split what we already have on https://github.com/rust-lang/triagebot/blob/master/src/bin/prioritization.rs into a set of things that can be executed using Zulip triagebot commands.

• Implement @triagebot commands for each step of our procedure
  • Unprioritized I-prioritize pending list
  • Unprioritized Regressions pending list
  • Nominations pending list
  • PRs waiting on team pending list
  • Agenda
    • Render MCP section only
    • Render Beta nominations only
    • Render Stable nominations only
    • Render PRs waiting on team only
    • Render Issues of note short summary only
    • Render Issues of note P-critical only
    • Render Issues of note Unassigned P-high regressions only
    • Render Nominated Issues only
    • Render the full agenda
  • Announcements
  • Toolstate
  • Performance regressions

Leaving this as is for now but we should be defining how the command should look like for each section and what would each command return.

[spastorino]

When implementing part of this we can put back parts of the code removed in rust-lang/triagebot#738 and rust-lang/triagebot#739