Restored _agkozak_vi_mode_indicator for legacy custom prompts

I have restored the _agkozak_vi_mode_indicator function as a legacy feature, as many people people use it in custom prompts. The default indicator can be expressed as '%(4V.:.%#)', though, and variations on this will be preferable to '$(_agkozak_vi_mode_indicator)', which entails a subshell.