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/assistant/src/slash_command/file_command.rs

@@ -58,7 +58,7 @@ impl FileSlashCommand {
                             .root_entry()
                             .map_or(false, |entry| entry.is_ignored),
                         include_root_name: true,
-                        directories_only: false,
+                        candidates: project::Candidates::Files,
                     }
                 })
                 .collect::<Vec<_>>();