Look for configuration files in $ZDOTDIR

Look for configuration files in $ZDOTDIR

This allows you to set/override the ZDOTDIR environment variable in your
~/.zshenv file to have different "sets" of AX-ZSH configurations in
different ZDOTDIR folders. For example, for different terminal emulator
applications, system architectures or hosts.

If ZDOTDIR is not set, $HOME is used (so no functional changes when the
ZDOTDIR environment variable is not set).