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/gpui/src/elements/list.rs

@@ -858,7 +858,7 @@ impl Styled for List {
 impl sum_tree::Item for ListItem {
     type Summary = ListItemSummary;
 
-    fn summary(&self) -> Self::Summary {
+    fn summary(&self, _: &()) -> Self::Summary {
         match self {
             ListItem::Unmeasured { focus_handle } => ListItemSummary {
                 count: 1,