[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[bot]]

⚠️ No Changeset found

Latest commit: bdc12167da3c229aa1b46bc9b44468746edbf75b

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[github-actions[bot]]

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

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