chore: add parallel to Suggests

`parallel::detectCores()` is used by `check_n_cores()` when
`n_cores = Inf`.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: billdenney

DESCRIPTION

@@ -57,6 +57,7 @@ Suggests:
     magick,
     methods,
     mirai,
+    parallel,
     pkgload (>= 1.0.2),
     quarto,
     rsconnect,