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/editor/src/display_map.rs

@@ -30,6 +30,7 @@ use crate::{
 pub use block_map::{
     BlockBufferRows, BlockChunks as DisplayChunks, BlockContext, BlockDisposition, BlockId,
     BlockMap, BlockPoint, BlockProperties, BlockStyle, RenderBlock, TransformBlock,
+    TransformBlockId,
 };
 use block_map::{BlockRow, BlockSnapshot};
 use collections::{HashMap, HashSet};