Make anchor_in_excerpt Optional (#8975)

We were seeing panics due to callers assuming they had valid
excerpt_ids, but that cannot easily be guaranteed across await points as
anyone may remove an excerpt.

Release Notes:

- Fixed a panic when hovering in a multibuffer

crates/language_tools/src/syntax_tree_view.rs

@@ -227,8 +227,12 @@ impl SyntaxTreeView {
         let multibuffer = editor_state.editor.read(cx).buffer();
         let multibuffer = multibuffer.read(cx).snapshot(cx);
         let excerpt_id = buffer_state.excerpt_id;
-        let range = multibuffer.anchor_in_excerpt(excerpt_id, range.start)
-            ..multibuffer.anchor_in_excerpt(excerpt_id, range.end);
+        let range = multibuffer
+            .anchor_in_excerpt(excerpt_id, range.start)
+            .unwrap()
+            ..multibuffer
+                .anchor_in_excerpt(excerpt_id, range.end)
+                .unwrap();
 
         // Update the editor with the anchor range.
         editor_state.editor.update(cx, |editor, cx| {