[2.x] SSR clustering

[RobertBoes]

Reattempt of #1909 for 2.x

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 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( ... ),
  {
    cluster: true,
  }
)

Massive thanks to @christianschoenmakers 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?
discord.com/channels/592327939920494592/758259460920573992/1255111006384820234