Allow SSR server clustering

[RobertBoes]

Currently the Inertia SSR server would only run on a single thread, so on high load this might have implications, since the Node process could only take 100% on a single core/thread.

Node has the cluster module https://nodejs.org/api/cluster.html, which would make it possible to start multiple Node servers on the same port, then each request would be dealt with by each thread in a round-robin way.

With this PR it's possible to enable clustering by using;

createServer(
  (page) => createInertiaApp( ... ),
  13714,
  true,
)

It's disabled by default. Kind of a downside of JS is that we can't use positional arguments, so you'd need to pass the port number along as well.

Massive thanks to ChristianQode who discovered the issue and asked about it on the Inertia.js Discord server:

Hi guys! I'm using Laravel Inertia + Vue with SSR. Sometimes the site is becoming really slow. I figured out this is because of the node SSR process is over 100% of CPU usage. However, this is only a single core, there are 7 cores idle. I want to make the node process capable of using multiple CPU cores to spread the load. What is the best way to do it? Should I spawn multiple processes with the inertia:start-ssr command, or should I implement node clustering somehow? Does anyone have a suggestion? https://discord.com/channels/592327939920494592/758259460920573992/1255111006384820234