Right now the `programs.zsh.syntax-highlighting.highlighters` option
lacks appropriate validation which can cause confusing things when
mistyping a higlighter for zsh-syntax-highlighting.
* programs.zsh: factor zsh-syntax-highlighting out into its own module
* programs.zsh.syntax-highlighting: add `highlighters` option
* programs.zsh: document BC break introduced by moving zsh-syntax-completion into its own module