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/events.ts

@@ -0,0 +1,25 @@
+import type { EmbedReplayData, TabData } from "./item";
+
+export type TabNavEvent = CustomEvent<{
+  data: TabData;
+  reload?: boolean;
+  replaceLoc?: boolean;
+  replayNotFoundError?: boolean;
+}>;
+
+export type RwpUrlChangeEvent = CustomEvent<
+  EmbedReplayData & {
+    type: "urlchange";
+    replayNotFoundError: boolean;
+  }
+>;
+
+export type ReplayLoadingDetail =
+  | { loading: true }
+  | { loading: false; replayNotFoundError: boolean };
+
+export type RwpPageLoadingEvent = CustomEvent<
+  Required<Pick<EmbedReplayData, "url" | "ts">> & {
+    type: "page-loading";
+  } & ReplayLoadingDetail
+>;