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/theme_selector/src/theme_selector.rs

@@ -9,7 +9,7 @@ use gpui::{
 };
 use parking_lot::Mutex;
 use postage::watch;
-use std::{cmp, sync::Arc};
+use std::{cmp, rc::Rc, sync::Arc};
 use theme::ThemeRegistry;
 use workspace::{Settings, Workspace};
 
@@ -64,14 +64,14 @@ impl ThemeSelector {
             Editor::single_line(
                 {
                     let settings = settings.clone();
-                    move |_| {
+                    Rc::new(move |_| {
                         let settings = settings.borrow();
                         EditorSettings {
                             tab_size: settings.tab_size,
                             style: settings.theme.selector.input_editor.as_editor(),
                             soft_wrap: editor::SoftWrap::None,
                         }
-                    }
+                    })
                 },
                 cx,
             )