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/file_finder/src/file_finder.rs

@@ -483,7 +483,7 @@ impl FileFinderDelegate {
                         .root_entry()
                         .map_or(false, |entry| entry.is_ignored),
                     include_root_name,
-                    directories_only: false,
+                    candidates: project::Candidates::Files,
                 }
             })
             .collect::<Vec<_>>();

crates/file_finder/src/new_path_prompt.rs

@@ -278,7 +278,7 @@ impl PickerDelegate for NewPathDelegate {
                         .root_entry()
                         .map_or(false, |entry| entry.is_ignored),
                     include_root_name,
-                    directories_only: true,
+                    candidates: project::Candidates::Directories,
                 }
             })
             .collect::<Vec<_>>();