Suggest language extensions for untitled buffers (#55263)

Fixes #53527.

## Summary

- Suggest `untitled.<extension>` when saving an untitled editor buffer
with a selected non-Plain Text language.
- Preserve the existing title-based suggestion for existing files, Plain
Text buffers, and buffers without a language extension.
- Add a regression test for an untitled Rust buffer suggesting
`untitled.rs`.

## Testing

- `mise exec rust@1.95.0 -- cargo fmt --check -p editor`
- `mise exec rust@1.95.0 -- cargo test -p editor
test_suggested_filename_uses_language_extension_for_untitled_buffer
--lib`

## Suggested .rules additions

None.

Release Notes:

- Fixed Save As suggestions for untitled buffers with a selected
language.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -2114,6 +2114,9 @@ impl MultiBuffer {
         self.title.as_deref()
     }
 
+    /// The title used for buffers not backed by a file and with no title of their own.
+    pub const DEFAULT_TITLE: &str = "untitled";
+
     pub fn title<'a>(&'a self, cx: &'a App) -> Cow<'a, str> {
         if let Some(title) = self.title.as_ref() {
             return title.into();
@@ -2131,7 +2134,7 @@ impl MultiBuffer {
             }
         };
 
-        "untitled".into()
+        Self::DEFAULT_TITLE.into()
     }
 
     fn buffer_content_title(&self, buffer: &Buffer) -> Option<Cow<'_, str>> {