Allow returning new requests from API routes

[frandiox]

Description

Exploring here a simple way to concatenate requests in an API route. This works with raw HTML forms. We could also return an RSC response instead, but we'd need a custom Form component to interpret the result.

Instead of returning a new Request we could use a helper function or anything like that.

@blittle @jplhomer do you think this concept could be interesting?

---

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• Read the Contributing Guidelines
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123)
• Update docs in this repository according to your change
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[blittle]

Holy cow, that turned out to be super simple. I was thinking this would be more complex. In my prior experimentation, I just returned a redirect. This is much better. Maybe when we implement the Form, we could also implement a helper for server components.

[jplhomer]

This is awesome! Very similar to what we've discussed in #883

[frandiox]

Ohh, I missed that conversation, but glad to see we are on the same page.
Should I clean this up to build on top of it? Or wait until we have a Form component that can also handle RSC responses?

[blittle]

@frandiox I think this could be useful independent of the Form component.

[blittle]

Great job!

[blittle]

So beautiful :)

[frandiox]

@mcvinci I'm merging this because we need to build on top of it but please, feel free to give feedback about the docs!

[mcvinci]

Thanks for updating the docs, @frandiox! I just have some minor suggestions regarding the codeblock syntax and some sentence tweaks to align with our style guide.

### Concatenating requests

You can concatenate requests in API routes to avoid extra network trips (waterfall requests). Concatenating requests is useful for HTML forms, where it's common to refresh the current page after the form submission is completed.

{% codeblock file, filename: "src/routes/my-page.server.jsx" %}

```jsx
export async function api(request) {
  if (request.method === 'POST') {
    // do some work here...
  }
  return new Request(request.url, {method: 'GET'});
}
export default function Page() {
  return (
    <form action="/my-page" method="POST">
      ...
    </form>
  );
}
```

{% endcodeblock %}

[mcvinci]

I've added a corresponding commit on the Shopify.dev side here: https://github.com/Shopify/shopify-dev/pull/18657/commits/82a68707be4c6a2c772b4bfbfd1467220b7db062