Fixes to issues with Dialog, FocusTrap, and StackContextProvider

* StackContextProvider had some reactivity that didn't make sense and would cause an infinite loop if it ever triggered.
* FocusTrap and Dialog did not work together properly all of the time. A top-level <Dialog> with initial state of closed would not have the FocusTrap triggered correctly when it was opened. Needed to make it reactive to changes in `containers`

src/lib/components/dialog/Dialog.svelte

@@ -253,10 +253,11 @@
   onUpdate={(message, element) => {
     return match(message, {
       [StackMessage.Add]() {
-        containers.add(element);
+        containers = new Set([...containers, element]);
       },
       [StackMessage.Remove]() {
         containers.delete(element);
+        containers = new Set([...containers]);
       },
     });
   }}