Implement support for launching child apps, and IPC between a parent and its children

[AtkinsSJ]

Details in commit messages.

See also https://github.com/HeyPuter/puter.js/pull/10 which implements the code that uses this!

There are a few questions to figure out:

• Should this functionality be part of launchApp instead of a special launchChildApp?
• What limitations do we want to place on launching child apps?
• What limitations do we want to place on apps communicating with each other?

Progress towards https://github.com/HeyPuter/puter/issues/220 :tada:

[KernelDeimos]

I think having them as two separate methods is fine for now because it's easy to merge that functionality later. I don't think we need any limitations [in terms of throughput] for now; I see that as more of a tuning thing for heavy workloads, so more like "what gets priority" than "what's the limit"

[KernelDeimos]

Actually let's have just launchApp doing it; since this is going in the SDK we need to support legacy function signatures so we'd be stuck with launchChildApp forever - better to just have one method

[AtkinsSJ]

Merged launchChildApp() into launchApp(). Launching an app is always treated as launching a child, and returns an AppConnection to it.