* fix: don't invoke asdf inside asdf commands
Recursive calls have a number of disadvantages:
* Poorer performance since each invocation spawns and new process and re-executes all the code in bin/asdf
* Makes debugging more difficult
* More likely to introduce subtle bugs and the possibility for infinite loops
Rename custom scripts for `plugin-add` and `plugin-remove` phases in plugins to make clear when they are executed:
`plugin-add` -> `post-plugin-add`
`plugin-remove` -> `pre-plugin-remove`