[Web] Capture sendmail stdin in remote Playground instances

[adamziel]

Adds sendmailSpawnHandler() (introduced in #3996) to PHP.wasm objects instantiated in the remote.html web worker. The handler catches any emails sent by WordPress and re-exposes it through an event listener as follows:

php.setCommandSpawnHandler('sendmail', sendmailSpawnHandler(php));

// In the website package:
php.addEventListener('sendmail.spawned', async (event) => {
	if (event.type !== 'sendmail.spawned') {
		return;
	}
	const rawMessage = await new Response(event.stdin).arrayBuffer();
	// Parse or relay rawMessage.
});

The goal is to expose these emails in the UI in a follow-up PR.