zsh: allow setting custom syntax highlighting styles (#4122)

* zsh: allow setting custom syntax highlighting styles

Custom styles allow overriding the default colors.
Example:
```nix
zsh.syntaxHighlighting.styles.comment = "fg=#6c6c6c";
```

See https://github.com/zsh-users/zsh-syntax-highlighting/blob/master/docs/highlighters.md

* zsh: allow configuring syntax-highlighting package

tests/modules/programs/zsh/default.nix

@@ -7,4 +7,5 @@
   zsh-history-ignore-pattern = ./history-ignore-pattern.nix;
   zsh-history-substring-search = ./history-substring-search.nix;
   zsh-prezto = ./prezto.nix;
+  zsh-syntax-highlighting = ./syntax-highlighting.nix;
 }