highlight array assignments of the form x[y]=...

This code is more lenient than bash.  Examples:

    $ x[y[]=
    zsh: no matches found: x[y[]=
    $ x[][]=
    zsh: no matches found: x[][]=

The proper solution is to look inside the [...] and make sure that all
unescaped/unquoted square brackes are matched, but that is a heck of
a lot more complicated than this simple 8-character patch.

highlighters/main/main-highlighter.zsh

@@ -153,7 +153,7 @@ _zsh_highlight_main_highlighter()
 _zsh_highlight_main_highlighter_check_assign()
 {
     setopt localoptions extended_glob
-    [[ $arg == [[:alpha:]_]([[:alnum:]_])#=* ]]
+    [[ $arg == [[:alpha:]_][[:alnum:]_]#(|\[*\])=* ]]
 }
 
 # Check if the argument is a path.