[Web][Networking] Call setupFetchNetworkTransport() in the web worker, not in remote.html - #2286

…, not in remote.html

This PR enables running simultaneous network calls asynchronously handled by multiple co-existing PHP
instances within the same worker.

The gist of the problem can be seen here:

```ts
const mgr = new PHPProcessManager({
	phpFactory: async () =>
		new PHP(await loadNodeRuntime(RecommendedPHPVersion)),
	maxPhpInstances: 3,
});

const primaryPhp = await mgr.getPrimaryPhp();

// We setup a JavaScript handler for WP_HTTP_Fetch by listening
// to its post_message_to_js() call via primaryPHP.onMessage()...
setupFetchNetworkTransport(primaryPhp);

// ...but that does nothing for the next spawned PHP instance. When
//    it calls post_message_to_js(), there's no message handler to
//    translate that into fetch(). As a result, the response comes
//    back empty.
const { php } = await mgr.acquirePHPInstance({ considerPrimary: false });
php.run({ code: "<?php wp_safe_remote_get('https://w.org'); " })
```

 ## Remaining work

A unit test or two would be nice.

 ## Testing instructions

1. Run this Blueprint: http://127.0.0.1:5400/website-server/?blueprint-url=https%3A%2F%2Fraw.githubusercontent.com%2Fautomattic%2Fremote-data-blocks-demo%2Ftrunk%2Fshopify%2Fshopify-blueprint.json
2. Click "Edit page"
3. Confirm the four requests to /wp-json/remote-data-blocks/v1/remote-data have actual data in their response body. Specifically, they should NOT have a JSON-encoded error message.