- 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.