Reintroduce mechanism for error annotations

[Keno]

We seem to have lost the capability to have the parser emit error annotations. That's a shame, because they were quite nice (e.g. in FancyDiagnostics). What would be the best way to add back that capability?

[ZacLN]

Can you use ParseState.errors? and possibly modify the Error to hold a type indicator field?

[Keno]

so what we want to do is have some place to store extra warning messages for every error (maybe just add an annotations array to the error type), emitting those when we unwind the stack after an error.