sum_tree: Store context on cursor (#34904)

This gets rid of the need to pass context to all cursor functions. In
practice context is always immutable when interacting with cursors.

A nicety of this is in the follow-up PR we will be able to implement
Iterator for all Cursors/filter cursors (hell, we may be able to get rid
of filter cursor altogether, as it is just a custom `filter` impl on
iterator trait).
Release Notes:

- N/A

crates/text/src/undo_map.rs

@@ -74,7 +74,6 @@ impl UndoMap {
                 undo_id: Default::default(),
             },
             Bias::Left,
-            &(),
         );
 
         let mut undo_count = 0;
@@ -99,7 +98,6 @@ impl UndoMap {
                 undo_id: Default::default(),
             },
             Bias::Left,
-            &(),
         );
 
         let mut undo_count = 0;