Convert Message::SyntaxError to use Diagnostic internally (#17784)

## Summary

This PR is a first step toward integration of the new `Diagnostic` type
into ruff. There are two main changes:
- A new `UnifiedFile` enum wrapping `File` for red-knot and a
`SourceFile` for ruff
- ruff's `Message::SyntaxError` variant is now a `Diagnostic` instead of
a `SyntaxErrorMessage`

The second of these changes was mostly just a proof of concept for the
first, and it went pretty smoothly. Converting `DiagnosticMessage`s will
be most of the work in replacing `Message` entirely.

## Test Plan

Existing tests, which show no changes.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>
Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_linter/src/message/github.rs

@@ -19,7 +19,7 @@ impl Emitter for GithubEmitter {
     ) -> anyhow::Result<()> {
         for message in messages {
             let source_location = message.compute_start_location();
-            let location = if context.is_notebook(message.filename()) {
+            let location = if context.is_notebook(&message.filename()) {
                 // We can't give a reasonable location for the structured formats,
                 // so we show one that's clearly a fallback
                 LineColumn::default()
@@ -43,7 +43,7 @@ impl Emitter for GithubEmitter {
             write!(
                 writer,
                 "{path}:{row}:{column}:",
-                path = relativize_path(message.filename()),
+                path = relativize_path(&*message.filename()),
                 row = location.line,
                 column = location.column,
             )?;