sum_tree: Do not implement Dimension on tuples, use new Dimensions wrapper instead (#35482)

This is a bit of a readability improvement IMHO; I often find myself
confused when dealing when dimension pairs, as there's no easy way to
jump to the implementation of a dimension for tuples to remind myself
for the n-th time how exactly that impl works. Now it should be possible
to jump directly to that impl.

Another bonus is that Dimension supports 3-ary tuples as well - by using
a () as a default value of a 3rd dimension.


Release Notes:

- N/A

crates/copilot/Cargo.toml

@@ -46,6 +46,7 @@ project.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 settings.workspace = true
+sum_tree.workspace = true
 task.workspace = true
 ui.workspace = true
 util.workspace = true