'main': Fix regression in zsh 5.3.1 and older: all precmd hooks later than z-sy-h would be aborted.

In those versions of zsh, «[[ -o nosuchoption ]]» is regarded as
a syntax error.  In newer zsh versions, it merely returns non-zero
(specifically, it returns 3, unlike «[[ -o unsetoption ]]» which
returns 1).

Fixes #732.
Fixes #733.

highlighters/main/main-highlighter.zsh

@@ -1692,7 +1692,7 @@ _zsh_highlight_main__precmd_hook() {
   # Unset the WARN_NESTED_VAR option, taking care not to error if the option
   # doesn't exist (zsh older than zsh-5.3.1-test-2).
   setopt localoptions
-  if [[ -o warnnestedvar ]] 2>/dev/null; then
+  if eval '[[ -o warnnestedvar ]]' 2>/dev/null; then
     unsetopt warnnestedvar
   fi