[unstable option] report_fixme

[scampi]

Tracking issue for report_fixme

[karyon]

Reopening, 2.0 is not happening and current docs are still pointing to this issue.

[calebcartwright]

Thanks @karyon. Couple minor additions/tweaks, I don't want to convey that 2.0 will never happen, it's just increasingly unlikely that it will happen outside an entire Rust ecosystem 2.0, and certainly going to happen any time soon, refs #5018

I think there's still a strong argument for us to drop this from rustfmt (it's quite orthogonal to formatting of code) and we may still do so

[calebcartwright]

Going to close this because stabilization doesn't make sense given our intent to deprecate and remove the option, refs #5102