Initializes asdf, installing it using git if not installed yet. Also, bypasses the shims if you're using the direnv plugin, as suggested by the plugin pro-tips.
The path of ASDF_DIR, where asdf is installed, is checked among the usual
directories created by Homebrew or Pacman, or the default ${HOME}/.asdf. You
can define ASDF_DIR, before where this module is initialized, to customize
it's path:
ASDF_DIR=/path/to/asdf_dir