[Mammoth] Add a call for listing a directory.

lib/mammoth/file/file.h

@@ -1,6 +1,9 @@
 #pragma once
 
+#include <glacier/container/vector.h>
 #include <glacier/memory/move.h>
+#include <glacier/status/error_or.h>
+#include <glacier/string/string.h>
 #include <glacier/string/string_view.h>
 
 #include "mammoth/util/memory_region.h"
@@ -29,4 +32,7 @@ class File {
       : file_data_(glcr::Move(file)), size_(size) {}
 };
 
+// TODO: Move this to a separate file.
+glcr::ErrorOr<glcr::Vector<glcr::String>> ListDirectory(glcr::StringView path);
+
 }  // namespace mmth