add window message notifications for embeds: (fixes #333)

[ikreymer]

• add 'replayNotFoundError' field to 'urlchange' (propagating 'archive-not-found' from wabac.js) to embed frame, with url/ts, for 404 pages
• propagate 'page-loading' with loading true/false, to indicate when a page started/finished loading

The page-loading message does not have the URL that is being loaded, as that is only available when loading is finished (via other messages like urlchange). The 'archive-not-found' message is initially sent from wabac.js to the immediate iframe, and then the 'replayNotFoundError' field is added to the urlchange event.

Only send one urlchange event per URL change (keep track of change).
Also ensure multi timestamp change uses replaceHistory to avoid an extra navigation.

Fixes #333

[ikreymer]

Message sequence for 404 page:

{type: 'page-loading', loading: true}
{type: 'page-loading', loading: false}
{type: 'urlchange', url: 'https://example.com/', ts: '20221002034558', 'replayNotFoundError': true}

For valid page:

{type: 'page-loading', loading: true}
{type: 'page-loading', loading: false}
{type: 'urlchange', url: 'https://example.com/', ts: '20221002034558', 'replayNotFoundError': false}

Only difference should be the replayNotFoundError field.

[SuaYoo]

What do you think about dispatching a wr-page-loading event from Embed? It may be more straightforward for <replay-web-page> embeds, to call like:

<replay-web-page
  replayBase="/js/"
  source="/wacz/example.wacz"
  url="https://webrecorder.net"
  @wr-page-loading=${handlePageLoading}
></replay-web-page>

[ikreymer]

What do you think about dispatching a wr-page-loading event from Embed? It may be more straightforward for <replay-web-page> embeds, to call like:

<replay-web-page
  replayBase="/js/"
  source="/wacz/example.wacz"
  url="https://webrecorder.net"
  @wr-page-loading=${handlePageLoading}
></replay-web-page>

yeah, that could work, though would be in addition to the postMessage messages (at least initially), since its sending between multiple window frames. I guess could have a @rwp-page-loading and @rwp-url-changed maybe?