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

crates/copilot/src/copilot.rs

@@ -39,6 +39,7 @@ use std::{
     path::{Path, PathBuf},
     sync::Arc,
 };
+use sum_tree::Dimensions;
 use util::{ResultExt, fs::remove_matching};
 use workspace::Workspace;
 
@@ -239,7 +240,7 @@ impl RegisteredBuffer {
                         let new_snapshot = new_snapshot.clone();
                         async move {
                             new_snapshot
-                                .edits_since::<(PointUtf16, usize)>(&old_version)
+                                .edits_since::<Dimensions<PointUtf16, usize>>(&old_version)
                                 .map(|edit| {
                                     let edit_start = edit.new.start.0;
                                     let edit_end = edit_start + (edit.old.end.0 - edit.old.start.0);