Define the helper functions for `bash-preexec.sh` immediately after importing it, rather than in `lib/theme`.
- `__check_precmd_conflict()` and `save_append_prompt_command()` are generally useful and not theme-specific.
- Add matching `__check_preexec_conflict()` and `safe_append_preexec()`.
Added a vendored lib loading routine in bash-it.sh
Added documentation on how to vendor libs in bash-it
Added and fixed plugins using preexec
Added tests for two plugins
Removed the old preexec lib