Adds the Stem Splitter plugin to clone_slopsmith()'s bundled-plugin
list so packaged builds ship it. The repo uses the feedBack-plugin-*
casing (capital B), so the lowercase prefix strip can't derive the
dirname — explicit :stem_splitter, matching the plugin id, same as
rig_builder.
Signed-off-by: topkoa <topkoa@gmail.com>