New features:
- borg_vfold_cv(): Guarded rsample::vfold_cv()
- borg_group_vfold_cv(): Guarded rsample::group_vfold_cv()
- borg_initial_split(): Guarded rsample::initial_split()
- borg_trainControl(): Guarded caret::trainControl()
- Hook system for automatic validation

Phase 5 complete. BORG is now feature-complete (90/100).