fish_user_paths variable is prepended to $PATH automatically. Some people set fish_user_paths, while others set PATH.
It will be better to test if fish_user_paths is set, and prepend it if it is, while just prepending to PATH otherwise.
- add `bin` and `shims` directories from asdf installation to PATH
- add `shims` directory from `$ASDF_DATA_DIR` (default `$HOME/.asdf`) to
PATH
- don't create directories in init script (directories are only added to
PATH if they exist, besides there is no warning when a directory does
not exist (perhaps it was a bug that was fixed))
This brings the fish snippet closer to the one for sh.
