Inline DiagnosticKind into other diagnostic types (#18074)

## Summary

This PR deletes the `DiagnosticKind` type by inlining its three fields
(`name`, `body`, and `suggestion`) into three other diagnostic types:
`Diagnostic`, `DiagnosticMessage`, and `CacheMessage`.

Instead of deferring to an internal `DiagnosticKind`, both `Diagnostic`
and `DiagnosticMessage` now have their own macro-generated `AsRule`
implementations.

This should make both https://github.com/astral-sh/ruff/pull/18051 and
another follow-up PR changing the type of `name` on `CacheMessage`
easier since its type will be able to change separately from
`Diagnostic` and `DiagnosticMessage`.

## Test Plan

Existing tests

crates/ruff_macros/src/lib.rs

@@ -49,7 +49,7 @@ pub fn derive_combine(input: TokenStream) -> TokenStream {
         .into()
 }
 
-/// Converts a screaming snake case identifier to a kebab case string.
+/// Converts an identifier to a kebab case string.
 #[proc_macro]
 pub fn kebab_case(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as syn::Ident);