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/multi_buffer/src/multi_buffer.rs

@@ -6,7 +6,7 @@ use clock::ReplicaId;
 use collections::{BTreeMap, Bound, HashMap, HashSet};
 use futures::{channel::mpsc, SinkExt};
 use git::diff::DiffHunk;
-use gpui::{AppContext, EventEmitter, Model, ModelContext};
+use gpui::{AppContext, EntityId, EventEmitter, Model, ModelContext};
 use itertools::Itertools;
 use language::{
     char_kind,
@@ -49,6 +49,12 @@ const NEWLINES: &[u8] = &[b'\n'; u8::MAX as usize];
 #[derive(Debug, Default, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
 pub struct ExcerptId(usize);
 
+impl From<ExcerptId> for EntityId {
+    fn from(id: ExcerptId) -> Self {
+        EntityId::from(id.0 as u64)
+    }
+}
+
 /// One or more [`Buffers`](Buffer) being edited in a single view.
 ///
 /// See <https://zed.dev/features#multi-buffers>
@@ -302,6 +308,7 @@ struct ExcerptBytes<'a> {
     reversed: bool,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum ExpandExcerptDirection {
     Up,
     Down,
@@ -4679,7 +4686,7 @@ impl ToPointUtf16 for PointUtf16 {
     }
 }
 
-fn build_excerpt_ranges<T>(
+pub fn build_excerpt_ranges<T>(
     buffer: &BufferSnapshot,
     ranges: &[Range<T>],
     context_line_count: u32,