[zsh] Clean up prompt
[zsh] Clean up prompt

- Remove unnecessary variables
- Add helper functions for color formatting
- Update

Color helpers h/t:
https://github.com/gabebw/dotfiles/blob/08e247ce7ec8e72500a917a05c2e22c80b413311/zshrc#L432-L440

reshleman/dotfiles,reshleman/dotfiles