This replaces things like `TypeCheckDiagnostic` with the new Diagnostic`
type.
This is a "surgical" replacement where we retain the existing API of
of diagnostic reporting such that _most_ of Red Knot doesn't need to be
changed to support this update. But it will enable us to start using the
new diagnostic renderer and to delete the old renderer. It also paves
the path for exposing the new `Diagnostic` data model to the broader Red
Knot codebase.