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/text/src/anchor.rs

@@ -136,6 +136,7 @@ where
 
 pub trait AnchorRangeExt {
     fn cmp(&self, b: &Range<Anchor>, buffer: &BufferSnapshot) -> Ordering;
+    fn intersects(&self, other: &Range<Anchor>, buffer: &BufferSnapshot) -> bool;
 }
 
 impl AnchorRangeExt for Range<Anchor> {
@@ -145,4 +146,8 @@ impl AnchorRangeExt for Range<Anchor> {
             ord => ord,
         }
     }
+
+    fn intersects(&self, other: &Range<Anchor>, buffer: &BufferSnapshot) -> bool {
+        self.start.cmp(&other.end, buffer).is_lt() && other.start.cmp(&self.end, buffer).is_lt()
+    }
 }