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/diagnostics/src/diagnostics_tests.rs

@@ -973,8 +973,8 @@ fn editor_blocks(
             blocks.extend(
                 snapshot
                     .blocks_in_range(DisplayRow(0)..snapshot.max_point().row())
-                    .enumerate()
-                    .filter_map(|(ix, (row, block))| {
+                    .filter_map(|(row, block)| {
+                        let transform_block_id = block.id();
                         let name: SharedString = match block {
                             TransformBlock::Custom(block) => {
                                 let mut element = block.render(&mut BlockContext {
@@ -984,7 +984,7 @@ fn editor_blocks(
                                     line_height: px(0.),
                                     em_width: px(0.),
                                     max_width: px(0.),
-                                    block_id: ix,
+                                    transform_block_id,
                                     editor_style: &editor::EditorStyle::default(),
                                 });
                                 let element = element.downcast_mut::<Stateful<Div>>().unwrap();