do not remove quotes when checking assignments

Zsh does not allow the variable name or the equals sign to be quoted or
escaped.  The previous code incorrectly highlighted the following
examples as assignments:

    $ 'x=y'
    zsh: command not found: x=y
    $ x\=y
    zsh: command not found: x=y
    $ "x"=y
    zsh: command not found: x=y
    $ \x=y
    zsh: command not found: x=y

highlighters/main/main-highlighter.zsh

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