Triagebot learns how to post a comment on GitHub to assign priority to an issue that is marked as regression. The command is sent from the Zulip Rust lang chat instance. The syntax is:

@triagebot prio #issue <priority>

example @triagebot prio #123456 high

The comment on GitHub is my usual comment on issue priority:

Assigning P-high (discussion on [Zulip](<zulip-link>)).

This handler checks that:

• issue number is really an issue (prevents assigning priority to PRs)
• the priority value is allowed

r? @Urgau

Thanks for a review!