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/Cargo.toml

@@ -18,11 +18,13 @@ collections.workspace = true
 ctor.workspace = true
 editor.workspace = true
 env_logger.workspace = true
+feature_flags.workspace = true
 futures.workspace = true
 gpui.workspace = true
 language.workspace = true
 log.workspace = true
 lsp.workspace = true
+multi_buffer.workspace = true
 project.workspace = true
 rand.workspace = true
 schemars.workspace = true