Allow custom timeouts

[deian]

I'm using the oclif generator (it's great, thanks!), but some of my commands are timing out when using the cli-ux flush implementation:

core/src/cli-ux/index.ts

Line 132 in 858b40c

await timeout(flush(), 10_000)

Any chance you'd be willing to take a PR with optional parameter?

ThankS!

[john-u]

This is related to #453 which impacted our project as well. We have a command that opens up an SSE connection for message streaming that was all of the sudden timing out after upgrading to newer versions of @oclif/core. It seems like commands that hit this issue violate an undocumented assumption from oclif: if the Node process is still busy after Command.run() resolves, something went wrong in the framework. Any command logic should either complete synchronously or await a Promise before the end of the run context. In this case, the flush function had a bug that made it not work and a recent PR fixed this which surfaced the issue for us and others. Here is an example of how we resolved it in our command.

Would it be worth updating the docs to point this out?