Look for typos when reporting an unknown nightly feature

[scrabsha]

r? @jdonszelmann

[scrabsha]

i am not happy of having two error types representing roughly the same error. using only UnknownFeature here means we need to be able to refer to Some<Ident> when creating the suggestion, which is (as far as i can tell) not a thing. i would love to be able to do the following:

#[derive(Diagnostic)]
#[diag(passes_unknown_feature, code = E0635)]
pub(crate) struct UnknownFeature {
    #[primary_span]
    pub span: Span,
    pub feature: Symbol,
    #[suggestion(
        style = "verbose",
        code = "{actual_name}",
        applicability = "maybe-incorrect"
    )]
    pub actual_name: Option<(Span, Symbol)>,
}

i'll try to see how feasible it is to add support for this ^ (or some variation of it).