editor: Support walking through overlapping diagnostics (#11139)

While looking into how to implement #4901, noticed that the current
`Goto next/previous diagnostic` behaved a bit weirdly. That is, when
there are multiple errors that have overlapping ranges, only the first
one can be chosen to be active by the `go_to_diagnostic_impl`.

### Previous behavior:


https://github.com/zed-industries/zed/assets/71292737/95897675-f5ee-40e5-869f-0a40066eb8e3

Doesn't go through all the diagnostics, and going backwards and forwards
doesn't show the same diagnostic always.

### New behavior:


https://github.com/zed-industries/zed/assets/71292737/81f7945a-7ad8-4a34-b286-cc2799b10500

Should always go through the diagnostics in a consistent manner.

Release Notes:
* Improved the behavioral consistency of "Go to Next/Previous
Diagnostic"

crates/language/src/buffer.rs

@@ -3159,7 +3159,21 @@ impl BufferSnapshot {
                 .iter_mut()
                 .enumerate()
                 .flat_map(|(ix, iter)| Some((ix, iter.peek()?)))
-                .min_by(|(_, a), (_, b)| a.range.start.cmp(&b.range.start))?;
+                .min_by(|(_, a), (_, b)| {
+                    let cmp = a
+                        .range
+                        .start
+                        .cmp(&b.range.start)
+                        // when range is equal, sort by diagnostic severity
+                        .then(a.diagnostic.severity.cmp(&b.diagnostic.severity))
+                        // and stabilize order with group_id
+                        .then(a.diagnostic.group_id.cmp(&b.diagnostic.group_id));
+                    if reversed {
+                        cmp.reverse()
+                    } else {
+                        cmp
+                    }
+                })?;
             iterators[next_ix].next()
         })
     }