Feat: async `exitOverride`

[cmackenthun]

While using this package, we have found a need to have better control of the error handling scenarios so that we can send observability signals upon errors. I attempted to use exitOverride, but found that I couldn't run async operations inside since the process is exited before the async function completes.

Not sure if exitOverride could be extended or if an exitOverrideAsync is needed, but it would allow us to wrap parse the error received, create observability signals, flush them, and then exit as normal

[shadowspawn]

Implementation detail: Commander calls process.exit() if the exitOverride callback returns, to terminate the normal program flow.

commander.js/lib/command.js

Lines 533 to 539 in 0692be5

	_exit(exitCode, code, message) {
	if (this._exitCallback) {
	this._exitCallback(new CommanderError(exitCode, code, message));
	// Expecting this line is not reached.
	}
	process.exit(exitCode);
	}

[shadowspawn]

Currently, I think you could use exitOverride() to get an exception for errors, catch the exception, and do the windup work there.

To allow an async callback, possibly Commander could wrap the callback promise, throw the wrapper and catch and return it from parseAsync(). The wrapper would throw an exception after the callback promise resolves so get "normal" exception exit.