feat: Full compliance with XDG Base Directory Specification

[hyperupcall]

Summary

This changes things so that:

• ASDF_CONFIG_FILE now defaults to $XDG_CONFIG_HOME/asdf/asdfrc, if the old location doesn't exist

• ASDF_DIR calculation is unchanged - only the installation procedures need to be changed

• ASDF_DATA_DIR now defaults to "$XDG_STATE_HOME/asdf", if the old location doesn't exist

  • Breaking change: If $HOME was empty, then this changes the location of the install directory to the new default. This shouldn't affect anyone - people without correct $HOME variables know what they are signing up for...

• Add undocumented arguments so things like completion scripts can reuse the logic without a whole bunch of code duplication

• Each environment variable is exported so that they new values are automatically used by plugins. If plugins use the environment variables, everything should Just Work

What still needs to be done:

• Documentation changes
• Installation instructions / script to update to use XDG_DATA_HOME (for the git repository) (for ASDF_DIR)
• Completion scripts to use the new location
  • completions/_asdf shows an example of how it should be done for Fish, Bash, etc. completions
• asdf.sh use the new location
• Fix weird issue with $ showing up in PATH when sourcing asdf.sh (this could just be me)

Fixes #687