Fix highlight z-index

Prevent the highlight from displaying above dropdowns opened after the
highlight is added. Display highlights, dropdowns, and modals in the
order they are added to the DOM.

mathesar_ui/src/App.svelte

@@ -138,7 +138,7 @@
     --modal-z-index: 1;
     --dropdown-z-index: 1;
     --cell-errors-z-index: 1;
-    --new-item-highlighter-z-index: 2;
+    --new-item-highlighter-z-index: 1;
     --toast-z-index: 3;
     --app-header-z-index: 1;