Add common issue templates and adjust triagebot configurations

[jieyouxu]

This PR adds some common issue templates, and updates some triagebot label configuration.

Common issue templates

As discussed in #t-rustfmt > Bug report template?, this PR adds 4 common issue templates to nudge the issue reporter to include sufficient info + reproducer on initial open, so that issues are more likely to be actionable from the very start (to save some manual nudging and also guide the issue reporter to produce effective issues).

1. General bug report. For general classes of bugs. There are two special cases which should instead use the following more specialized templates...
2. Internal Compiler Error (ICE). This is separate since ICE will also have backtraces.
3. Regressions. This is separate because these should receive regression-* + I-prioritization labels, since regressions have some degree of urgency and needs impact assessment / prioritization.
4. Feature requests. A separate template, which includes a reminder about the core mission/scope of rustfmt versus auxiliary configuration options being necessarily secondary / less prioritized.

Triagebot configuration updates

In support of the issue templates, this PR also adjusts triagebot to:

• Allow users without write access to adjust regression-* labels on issues via @rustbot label.
• Automatically attach I-prioritize label to regression-* issues if they do not already have priority P-* labels (or if the issue requires nightly).
  • As a follow-up, we can also setup a prioritization Zulip channel, if needed, like #t-compiler/prioritization/alerts, which I-prioritize label will automatically create.

r? @ytmimi