fix: URL instance methods work in load

[elliott-with-the-longest-name-on-github]

Fixes #5083.

This is the "surgical approach" -- just replace the hash property with a custom getter. The other option is (if we're really committed to using a Proxy):

new Proxy(url, {
    get: (target, property, receiver) => {
        if (property === 'hash') {
            throw new Error(
                'url.hash is inaccessible from load. Consider accessing hash from the page store within the script tag of your component.'
            );
        }

        if (!(property in target)) {
          return undefined;
        }
        const value = target[property];
        return typeof value === "function"
          ? (...args) => value.apply(target, args)
          : value;
    }
});

This scares me a little bit, because it's a lot harder to anticipate what other side effects it might exhibit.

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[Rich-Harris]

Thank you — this is definitely the right approach, rather than making the proxy more sophisticated — confirmed at excruciating length via Twitter.

I made a couple of small tweaks:

• using a LoadURL subclass instead of monkey patching — feels less naughty, somehow
• if someone wasn't using destructuring, we'd want event.url === event.url to be true, so create the LoadURL instance eagerly and return it from the getter
• use it for SSR as well as client-side navigation, so that the error is consistent

It occurred to me that we probably want to do something similar for the prerendering proxy (the one that blocks use of url.search[Params]), but I left that out for now just to make life easy

[elliott-with-the-longest-name-on-github]

Ugh, thanks for that Twitter thread. After reading a few dozen subthreads, I think I'm starting to understand why the proxy approach doesn't work: It's because JavaScript is stupid. 🤣

If we want to implement similar behavior for prerendering, I'm happy to tackle that. I can do it as part of this PR or as a separate one after this one is merged.