(enh) add OS / Process.exit

[joshgoebel]

Changes:

• allows immediately terminating the running script and setting the result code

This is needed for usage with terminal scripting, CI (test suites), build systems, etc. Being able to return an error code (without purposely crashing) is necessary for many different uses cases.

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I think this is sufficient because freeVm() does all the
cleanup calling ioShutdown and schedulerShutdown(). What
I'm not 100% sure of is if I need to address this:

Then, after uv_run returns you can use uv_walk to close
all handles and run the loop one last time so that
close callbacks are called.

If so I think I can add it fairly easily. I've fully implemented closing all handles now.

Ref: libuv/libuv#709

[joshgoebel]

Updated: Attempts to properly clean up fully before stopping completely. This also improves scheduler error handling which also uses uv_stop to hard stop which may not fully clean-up...

[joshgoebel]

Converting to draft, seems obvious now this needs more work.

[joshgoebel]

Fairly confident this works:

joshgoebel@1de80e5

The flow:

• Wren VM calls into C for Process.exit_
• uv_stop() - stop the UV event loop
• Control returns to Wren VM
• Fiber.suspend - to stop the VM also
• VM is halted and CLI terminates because there is no more work to do

I can update my patch here if there is agreement this is a viable solution.