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/editor/src/display_map/block_map.rs

@@ -1360,7 +1360,7 @@ impl<'a> Iterator for BlockBufferRows<'a> {
 impl sum_tree::Item for Transform {
     type Summary = TransformSummary;
 
-    fn summary(&self) -> Self::Summary {
+    fn summary(&self, _cx: &()) -> Self::Summary {
         self.summary.clone()
     }
 }