Add an experimental, WIP diagnostics grouping panel (#14515)

Provide a current, broken state as an experimental way to browse
diagnostics.
The diagnostics are grouped by lines and reduced into a block that, in
case of multiple diagnostics per line, could be toggled back and forth
to show more diagnostics on the line.
Use `grouped_diagnostics::Deploy` to show the panel.

Issues remaining:
* panic on warnings toggle due to incorrect excerpt manipulation
* badly styled blocks
* no key bindings to navigate between blocks and toggle them
* overall odd usability gains for certain groups of people

Due to all above, the thing is feature-gated and not exposed to regular
people.


Release Notes:

- N/A

crates/project/src/project.rs

@@ -11720,7 +11720,7 @@ fn sort_search_matches(search_matches: &mut Vec<SearchMatchCandidate>, cx: &AppC
     });
 }
 
-fn compare_paths(
+pub fn compare_paths(
     (path_a, a_is_file): (&Path, bool),
     (path_b, b_is_file): (&Path, bool),
 ) -> cmp::Ordering {