Group diagnostics by primary

Render primary message above the excerpt and supporting messages as block decorations with a `Below` disposition. This is still super rough.

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/editor/src/multi_buffer/anchor.rs

@@ -14,6 +14,13 @@ pub struct Anchor {
 }
 
 impl Anchor {
+    pub fn new(excerpt_id: ExcerptId, text_anchor: text::Anchor) -> Self {
+        Self {
+            excerpt_id,
+            text_anchor,
+        }
+    }
+
     pub fn min() -> Self {
         Self {
             excerpt_id: ExcerptId::min(),