driver: Track rename of an unreleased upstream API.

ISEARCH_ACTIVE was renamed ISEARCHMATCH_ACTIVE in workers/38921
(commit f06be0ffcf2c to zsh itself).

Fixes #299.

zsh-syntax-highlighting.zsh

@@ -121,7 +121,7 @@ _zsh_highlight()
     (( $+YANK_ACTIVE )) && (( YANK_ACTIVE )) && _zsh_highlight_apply_zle_highlight paste standout "$YANK_START" "$YANK_END"
 
     # isearch
-    (( $+ISEARCH_ACTIVE )) && (( ISEARCH_ACTIVE )) && _zsh_highlight_apply_zle_highlight isearch underline "$ISEARCH_START" "$ISEARCH_END"
+    (( $+ISEARCHMATCH_ACTIVE )) && (( ISEARCHMATCH_ACTIVE )) && _zsh_highlight_apply_zle_highlight isearch underline "$ISEARCHMATCH_START" "$ISEARCHMATCH_END"
 
     # suffix
     (( $+SUFFIX_ACTIVE )) && (( SUFFIX_ACTIVE )) && _zsh_highlight_apply_zle_highlight suffix bold "$SUFFIX_START" "$SUFFIX_END"