[iso] Add async onBeforeRoute() prop hook

[developit]

This implements <Router onBeforeRoute={async url => {}}>. When onBeforeRoute returns a Promise for a given URL transition, that Promise is thrown from within the incoming route to simulate a Suspense-based paused transition. This means each router maintains its own onBeforeRoute "locking" state.

Here's an example that uses onBeforeRoute to fetch metadata before rendering any routes:

import { useCallback, useEffect } from 'preact/hooks';
import { LocationProvider, Router, hydrate } from 'preact-iso';

function App() {
  const [meta, setMeta] = useState({});
  const loadMeta = useCallback(async url => {
    const res = await fetch(`${url}.json`);
    setMeta(await res.json());
  }, []);
  useEffect(() => {
    document.title = meta.title;
  }, [meta.title]);
  return (
    <LocationProvider>
      <Router onBeforeRoute={loadMeta}>
        <Home path="/" meta={meta} />
        <About path="/about" meta={meta} />
      </Router>
    </LocationProvider>
  );
}

hydrate(<App />);

Demo:
https://glitch.com/edit/#!/plausible-satin-rat?path=public%2Fapp.js%3A44%3A5

[changeset-bot]

⚠️ No Changeset found

Latest commit: bdc1216

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[github-actions]

Size Change: +188 B (+3%)

Total Size: 6.5 kB

Filename	Size	Change
packages/preact-iso/.dist/index.********.js	1.78 kB	+70 B (+4%)
packages/preact-iso/.dist/prerender.********.js	348 B	-1 B (0%)
packages/preact-iso/router.js	2.66 kB	+119 B (+5%)	🔍

ℹ️ View Unchanged

Filename	Size
packages/preact-iso/hydrate.js	290 B
packages/preact-iso/index.js	195 B
packages/preact-iso/lazy.js	594 B
packages/preact-iso/prerender.js	629 B

_{compressed-size-action}