Co-authored-by: Josh Soref <jsoref@users.noreply.github.com>

Initializes jenv and provides the jenv_prompt_info funtion to add
Java version information to prompts.  This function is stubbed in
prompt_info_functions script to allow it to be safely called
regardless of whether or not the jenv plugin is loaded.

It also splits detection of the plugin/versions directory and bin directory
to suppport the way Homebrew splits the jenv bin and data directories