feat: Add preload() method to lazy

[rschristian]

Still need to figure out the right song & dance for sinon to play nice, but would allow for easier preloading.

import { render } from 'preact';
import { LocationProvider, Router, Route, lazy } from 'preact-iso';

const NotFound = lazy(() => import('./pages/_404.jsx'));

function Home() {
    return (
        <a href="/404" onMouseOver={() => NotFound.preload()}>
            Not Found -- Hover me to preload `NotFound`!
        </a>
    );
}

function App() {
    return (
        <LocationProvider>
            <Router>
                <Route path="/" component={Home} />
                <Route default component={NotFound} />
            </Router>
        </LocationProvider>
    )
}

render(<App />, document.getElementById('app'));