Automatically create tracking issues when major change are accepted

[Urgau]

This PR adds the required infrastructure to automatically create a tracking issues when a major change is accepted.

[major-change.tracking-issue-template]
repository = "triagebot"
title = "Tracking issue for MCP#${mcp_number}"
body = """
Multi line body with ${mcp_number} and ${mcp_title}
"""
labels = ["C-tracking-issue", "T-compiler"]

Fixes #2067
cc @fmease

[Kobzol]

Implementation looks good!

View changes since this review

[fmease]

This looks great, thanks!

I don't want to smother you with extra work, so these things can be done as follow-ups (or not all :P): In my issue & MCP I also proposed to

• have some suppress-auto-tracking label to suppress the creation of the tracking issue.

  So it would be pretty great if one was able to register such a label in [major-change.tracking-issue-template] assuming you agree that such a mechanism would be useful (IIRC @Kobzol doubted the usefulness).

  That could come in clutch in situations like open MCP Remove diagnostic translation infrastructure compiler-team#924 where we already have a tracking issue: Tracking Issue for rustc's translatable diagnostics infrastructure rust#132181 (sort of). And IIRC my MCP / issue had some other vague examples?

• have a way to ping the MCP author (and registered mentors if technically feasible) since the link to the MCP can be easily missed; however, it's probably not worth the effort

[Urgau]

• have some suppress-auto-tracking label to suppress the creation of the tracking issue

While I don't doubt the usefulness in some scenario I think they are going to be extremely limited. Even in your example I would still want to have the tracking issue created as they wouldn't track the same thing.

If we see the need for it we can always add it later.

• have a way to ping the MCP author

Done, added the ${mcp_author} variable to get the handle of the MCP issue author.

• and registered mentors if technically feasible

That would require some sort of easily-parseble format for triagebot, it's currently a free for all. Let's defer that for now.