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_source_file/src/lib.rs

@@ -195,7 +195,7 @@ impl SourceFile {
         }
     }
 
-    fn index(&self) -> &LineIndex {
+    pub fn index(&self) -> &LineIndex {
         self.inner
             .line_index
             .get_or_init(|| LineIndex::from_source_text(self.source_text()))