feat: Add "not found" error detail on page load (#482)

Resolves https://github.com/webrecorder/replayweb.page/issues/481 with
workaround
Related to https://github.com/webrecorder/browsertrix/issues/2865

## Changes

- Includes `replayNotFoundError` in `"page-loading"` event when page is
finished loading as a workaround for the actual `replayNotFoundError`
value not being available on `urlchange`.
- Minor refactor to more strongly typed events.

src/index.ts

@@ -58,7 +58,11 @@ export {
 
 export type { ItemType, URLResource } from "./types";
 
-export type { EmbedReplayEvent } from "./item";
+export type {
+  RwpUrlChangeEvent as EmbedReplayEvent,
+  RwpUrlChangeEvent,
+  RwpPageLoadingEvent,
+} from "./events";
 
 export * from "./misc";