feat: improve fresh async render to render after/during hydration

[igorbrasileiro]

Describe the feature request

Improve the user experience, and core web vitals metrics(FCP, LCP), rendering the async render section immediately after/during the fresh hydration.

Current behavior

Today the fresh LoadingFallback fetches the async rendered section in two scenarios:

1. when readyState === complete;
2. otherwise, when the event load is fired

The problem happens in the second case, because, sometimes, the page takes too long to trigger the load event due to third-party scripts.

Expected behavior

Render the async render section immediately after/during the fresh hydration

[igorbrasileiro]

A proposal: add a data-attribute (data-partial-trigger or data-async-render-trigger) to the fresh binding LoadingFallback button that triggers the partial.

By doing this, an island in the userland can add a query for these button triggers and click on them.

[igorbrasileiro]

I've changed the proposal to a better approach that switches the current way to trigger the fresh partial, which uses a script with a 'load' event listener, to an island that triggers the partial. Islands are better because it always runst before the load event, delivering a better UX for those whos are navigating on those sites