assistant: Add diagnostics slash command (#12998)

This adds a `diagnostics` command to the assistant which allows to
inject compile errors/warnings into the context.

Release Notes:

- N/A

crates/project/src/project_tests.rs

@@ -765,7 +765,7 @@ async fn test_reporting_fs_changes_to_language_servers(cx: &mut gpui::TestAppCon
             worktree
                 .read(cx)
                 .snapshot()
-                .entries(true)
+                .entries(true, 0)
                 .map(|entry| (entry.path.as_ref(), entry.is_ignored))
                 .collect::<Vec<_>>(),
             &[
@@ -839,7 +839,7 @@ async fn test_reporting_fs_changes_to_language_servers(cx: &mut gpui::TestAppCon
             worktree
                 .read(cx)
                 .snapshot()
-                .entries(true)
+                .entries(true, 0)
                 .map(|entry| (entry.path.as_ref(), entry.is_ignored))
                 .collect::<Vec<_>>(),
             &[