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

index.d.ts

@@ -5,6 +5,7 @@ declare module "*.scss" {
 
 // dom custom event
 interface CustomEventMap {
+  prenav: CustomEvent<{}>
   nav: CustomEvent<{ url: FullSlug }>
   themechange: CustomEvent<{ theme: "light" | "dark" }>
 }