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/feature_flags/src/feature_flags.rs

@@ -34,6 +34,11 @@ impl FeatureFlag for TerminalInlineAssist {
     const NAME: &'static str = "terminal-inline-assist";
 }
 
+pub struct GroupedDiagnostics {}
+impl FeatureFlag for GroupedDiagnostics {
+    const NAME: &'static str = "grouped-diagnostics";
+}
+
 pub trait FeatureFlagViewExt<V: 'static> {
     fn observe_flag<T: FeatureFlag, F>(&mut self, callback: F) -> Subscription
     where