Add Buffer::outline method

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/editor/src/multi_buffer.rs

@@ -7,7 +7,7 @@ use collections::{HashMap, HashSet};
 use gpui::{AppContext, Entity, ModelContext, ModelHandle, Task};
 use language::{
     Buffer, BufferChunks, BufferSnapshot, Chunk, DiagnosticEntry, Event, File, Language, Selection,
-    ToOffset as _, ToPoint as _, TransactionId,
+    ToOffset as _, ToPoint as _, TransactionId, Outline,
 };
 use std::{
     cell::{Ref, RefCell},
@@ -1698,6 +1698,10 @@ impl MultiBufferSnapshot {
             })
     }
 
+    pub fn outline(&self) -> Option<Outline> {
+        self.as_singleton().and_then(move |buffer| buffer.outline())
+    }
+
     fn buffer_snapshot_for_excerpt<'a>(
         &'a self,
         excerpt_id: &'a ExcerptId,