Go over all structured parser suggestions and make them verbose style.
When suggesting to add or remove delimiters, turn them into multiple suggestion parts.
Spruce up the diagnostics of some early lints
Implement the various "*(note to myself) in a follow-up PR we should turn parts of this message into a subdiagnostic (help msg or even struct sugg)*" drive-by comments I left in #124417 during my review.
For context, before #124417, only a few early lints touched/decorated/customized their diagnostic because the former API made it a bit awkward. Likely because of that, things that should've been subdiagnostics were just crammed into the primary message. This PR rectifies this.
The check previously matched this, and suggested adding a missing
`struct`:
pub Foo(...):
It was probably intended to match this instead (semicolon instead of
colon):
pub Foo(...);
