driver: Enable highlighting during isearch under zsh≥5.3.

This patch causes a behaviour difference in the [i257] scenario:

- Before this change, the zle_highlight[isearch] is applied and z-sy-h's
  highlighting isn't.

- With this change, both zle_highlight[isearch] and z-sy-h's
  highlighting are applied, so «echo foo» renders the first word in green
  underline (fg=green from ZSH_HIGHLIGHT_STYLES[builtin], underline from
  zle_highlight[isearch]).

This patch causes the presuppositional FAQ entry added in
a8fe22d422 to be correct.

This is part of #261, of which #288 was a spin-off.

[i257] https://github.com/zsh-users/zsh-syntax-highlighting/pull/257#issuecomment-168394665

zsh-syntax-highlighting.zsh

@@ -60,7 +60,7 @@ _zsh_highlight()
 
   # Remove all highlighting in isearch, so that only the underlining done by zsh itself remains.
   # For details see FAQ entry 'Why does syntax highlighting not work while searching history?'.
-  if [[ $WIDGET == zle-isearch-update ]]; then
+  if [[ $WIDGET == zle-isearch-update ]] && ! (( $+ISEARCHMATCH_ACTIVE )); then
     region_highlight=()
     return $ret
   fi