Show diagnostics in scrollbar (#7175)

This PR implements support for displaying diagnostics in the scrollbar,
similar to what is already done for search results, symbols, git diff,
...

For example, changing a field name (`text`) without changing the
references looks like this in `buffer.rs` (note the red lines in the
scrollbar):

![image](https://github.com/zed-industries/zed/assets/53836821/c46f0d55-32e3-4334-8ad7-66d1578d5725)

As you can see, the errors, warnings, ... are displayed in the scroll
bar, which helps to identify possible problems with the current file.

Relevant issues: #4866, #6819

Release Notes:

- Added diagnostic indicators to the scrollbar

crates/multi_buffer/src/multi_buffer.rs

@@ -3052,6 +3052,12 @@ impl MultiBufferSnapshot {
         self.has_conflict
     }
 
+    pub fn has_diagnostics(&self) -> bool {
+        self.excerpts
+            .iter()
+            .any(|excerpt| excerpt.buffer.has_diagnostics())
+    }
+
     pub fn diagnostic_group<'a, O>(
         &'a self,
         group_id: usize,