Adds support for AsyncStream

[mcritz]

This PR:

• Adds AsyncStream support to FileMonitor for modern Swift Structured Concurrency support
• Bumps the Package build requirement to 5.9, breaking backward compatibility

Benefits:
Users can change callback based system with for await event in fileMonitor.stream { ... }

Drawbacks:
I have forced the Package to use Swift 5.9 instead of using #available or @available. We could use an availability check to avoid breaking backward compatibility. OR we could release a new, breaking release of this Package using Github releases.

I’m open to either approach, but we should pick one.

[mcritz]

@petershaw Any interest in taking this change?

[jagreenwood]

I'm in favor of offering a stream to consume events.
What would you think about a method that returns the stream (and sets an optional continuation member) rather than initialing the stream by default?

[tklae]

Thanks for adding this and sorry for the late reply, somehow we missed it... We will have a look and get back to you, soon!

[KrisSimon]

Hey @mcritz
I am totally fine with a change to the swift 5.9 dependency!

I'll manage to take a look to the implementation at the beginning of next week.
Pretty cool at the first look! Thank you very much for contribution.

[KrisSimon]

On Linux the FileMonitorTests.testLifecycleChange hangs.
@mcritz can you take a look into it?

[KrisSimon]

@mcritz I split test into two and fixed the problem.
Let's wait for the CI confirmation. ;)

[mcritz]

@KrisSimon Thanks for fixing that. Happy to have made a contribution!

[KrisSimon]

@mcritz thank you very much for your contribution.