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/renderer/display_list.rs

@@ -909,6 +909,7 @@ pub(crate) enum DisplayAnnotationType {
 impl From<snippet::Level> for DisplayAnnotationType {
     fn from(at: snippet::Level) -> Self {
         match at {
+            snippet::Level::None => DisplayAnnotationType::None,
             snippet::Level::Error => DisplayAnnotationType::Error,
             snippet::Level::Warning => DisplayAnnotationType::Warning,
             snippet::Level::Info => DisplayAnnotationType::Info,