assistant2: Rework @mentions (#26983)

https://github.com/user-attachments/assets/167f753f-2775-4d31-bfef-55565e61e4bc

Release Notes:

- N/A

crates/project/src/project.rs

@@ -390,6 +390,8 @@ pub struct Completion {
     pub documentation: Option<CompletionDocumentation>,
     /// Completion data source which it was constructed from.
     pub source: CompletionSource,
+    /// A path to an icon for this completion that is shown in the menu.
+    pub icon_path: Option<SharedString>,
     /// An optional callback to invoke when this completion is confirmed.
     /// Returns, whether new completions should be retriggered after the current one.
     /// If `true` is returned, the editor will show a new completion menu after this completion is confirmed.