Use the new, unreleased zsh 'memo=' feature to remove only our own entries from $region_highlight.

Fixes #418 (interoperability issue with other plugins).

tests/test-highlighting.zsh

@@ -190,7 +190,7 @@ run_test_internal() {
     if
       [[ $start != $exp_start ]] ||
       [[ $end != $exp_end ]] ||
-      [[ $highlight_zone[3] != $expected_highlight_zone[3] ]]
+      [[ ${highlight_zone[3]%,} != ${expected_highlight_zone[3]} ]] # remove the comma that's before the memo field
     then
       print -r -- "not ok $i - $desc - expected ($exp_start $exp_end ${(qqq)expected_highlight_zone[3]}), observed ($start $end ${(qqq)highlight_zone[3]}). $todo"
       if [[ -z $todo ]]; then (( ++print_expected_and_actual )); fi