Start on adding support for editing via the assistant panel (#14795)

Note that this shouldn't have any visible user-facing behavior yet. The
feature is incomplete but we wanna merge early to avoid a long-running
branch.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/language/src/outline.rs

@@ -23,6 +23,7 @@ pub struct OutlineItem<T> {
     pub text: String,
     pub highlight_ranges: Vec<(Range<usize>, HighlightStyle)>,
     pub name_ranges: Vec<Range<usize>>,
+    pub body_range: Option<Range<T>>,
 }
 
 impl<T> Outline<T> {