perf(explorer): client side explorer (#1810)

* start work on client side explorer

* fix tests

* fmt

* generic test flag

* add prenav hook

* add highlight class

* make flex more consistent, remove transition

* open folders that are prefixes of current path

* make mobile look nice

* more style fixes

docs/advanced/creating components.md

@@ -161,6 +161,18 @@ document.addEventListener("nav", () => {
 })
 ```
 
+You can also add the equivalent of a `beforeunload` event for [[SPA Routing]] via the `prenav` event.
+
+```ts
+document.addEventListener("prenav", () => {
+  // executed after an SPA navigation is triggered but
+  // before the page is replaced
+  // one usage pattern is to store things in sessionStorage
+  // in the prenav and then conditionally load then in the consequent
+  // nav
+})
+```
+
 It is best practice to track any event handlers via `window.addCleanup` to prevent memory leaks.
 This will get called on page navigation.