Fix: z-index for codemirror hint not large enough (#4551)

It is currently covered by other tiddler.

plugins/tiddlywiki/codemirror-autocomplete/files/addon/hint/show-hint.css

@@ -1,6 +1,6 @@
 .CodeMirror-hints {
   position: absolute;
-  z-index: 10;
+  z-index: 999;
   overflow: hidden;
   list-style: none;