Clean up highlight.zsh
Clean up highlight.zsh

Albeit the overall resulting prompt remains unchanged, highlight.zsh has
been massively updated to be more in keeping with nicer shell scripting
principles.

Some of these changes include:

- improvements to readability (case in point: the massive one-liner
  parameter expansion for collapse_pwd is now entirely refactored into
  an easy-to-follow algorithm)
- less reliance on external tools and/or non-standard flags for such
  tools
- better use of zsh idioms, notably parameter expansion flags and
  conditional prompt expressions
- generous reduction of superfluous lines of code

igemnace/zsh-config