Improve same line diagnostic rendering (#14741)

Follow-up of https://github.com/zed-industries/zed/pull/14515

Fixed certain visual artifacts, related to multi line diagnostics and
block toggle rendering.
Also enabled diagnostics toolbar controls for the experimental view too.


Release Notes:

- N/A

crates/diagnostics/src/diagnostics.rs

@@ -4,7 +4,7 @@ mod toolbar_controls;
 
 #[cfg(test)]
 mod diagnostics_tests;
-mod grouped_diagnostics;
+pub(crate) mod grouped_diagnostics;
 
 use anyhow::Result;
 use collections::{BTreeSet, HashSet};