ruff_annotate_snippets: make small change to enable omitting header

This is a tiny change that, perhaps slightly shady, permits us to use
the `annotate-snippets` renderer without its mandatory header (which
wasn't there in `annotate-snippets 0.9`). Specifically, we can now do
this:

    Level::None.title("")

The combination of a "none" level and an empty label results in the
`annotate-snippets` header being skipped entirely. (Not even an empty
line is written.)

This is maybe not the right API for upstream `annotate-snippets`, but
it's very easy for us to do and unblocks the upgrade (albeit relying on
a vendored copy).

Ref https://github.com/rust-lang/annotate-snippets-rs/issues/167

crates/ruff_annotate_snippets/src/snippet.rs

@@ -126,6 +126,8 @@ impl<'a> Annotation<'a> {
 /// Types of annotations.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum Level {
+    /// Do not attach any annotation.
+    None,
     /// Error annotations are displayed using red color and "^" character.
     Error,
     /// Warning annotations are displayed using blue color and "-" character.