highlighters: Use _zsh_highlight_add_highlight

_zsh_highlight_add_highlight appends to region_highlight $1 $2 and the
next non-null parameter. If there is no non-null parameter, do nothing,
This is so that highlighters can add a style with fallbacks if the most
specific style is not defined. If none of the applicable styles are
defined, do the right thing and don't add an invalid entry to
region_highlight.

The pattern highlighter doesn't use this function as it'd need too large
of an API change.

highlighters/brackets/brackets-highlighter.zsh

@@ -83,10 +83,10 @@ _zsh_highlight_brackets_highlighter()
       local bracket_color_size=${#ZSH_HIGHLIGHT_STYLES[(I)bracket-level-*]}
       local bracket_color_level=bracket-level-$(( (levelpos[$pos] - 1) % bracket_color_size + 1 ))
       local style=$bracket_color_level
-      region_highlight+=("$pos $((pos + 1)) $ZSH_HIGHLIGHT_STYLES[$style]")
+      _zsh_highlight_add_highlight $pos $((pos + 1)) $style
     else
       local style=bracket-error
-      region_highlight+=("$pos $((pos + 1)) $ZSH_HIGHLIGHT_STYLES[$style]")
+      _zsh_highlight_add_highlight $pos $((pos + 1)) $style
     fi
   done
 
@@ -95,7 +95,7 @@ _zsh_highlight_brackets_highlighter()
   if [[ -n $levelpos[$pos] ]] && [[ -n $matching[$pos] ]]; then
     local otherpos=$matching[$pos]
     local style=cursor-matchingbracket
-    region_highlight+=("$otherpos $((otherpos + 1)) $ZSH_HIGHLIGHT_STYLES[$style]")
+    _zsh_highlight_add_highlight $otherpos $((otherpos + 1)) $style
   fi
 }