Pass Summary::Context to Item::summarize (#18510)

We are going to use this in the multi-buffer to produce a summary for an
`Excerpt` that contains a `Range<Anchor>`.

Release Notes:

- N/A

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

crates/rope/src/rope.rs

@@ -1159,7 +1159,7 @@ impl Chunk {
 impl sum_tree::Item for Chunk {
     type Summary = ChunkSummary;
 
-    fn summary(&self) -> Self::Summary {
+    fn summary(&self, _cx: &()) -> Self::Summary {
         ChunkSummary::from(self.0.as_str())
     }
 }