Track Libs MCPs in triage

[KodrAus]

We don't have an explicit process for MCPs in Libs yet, but I've just got one open as a PR (#6) with a mcp tag. We could fetch these and include them in our agenda generator if we wanted to keep regular tabs on them.

[m-ou-se]

@joshtriplett Assigning this to you, since you wanted to set up the stuff we need to do MCPs if I remember correctly.

[m-ou-se]

To do: add triagebot.toml to this repo

[Mark-Simulacrum]

See https://github.com/rust-lang/compiler-team/blob/master/triagebot.toml for an example wrt to MCP creation. Happy to help answer questions but want someone else to do it so we build out muscle for it :)