Fix: Move platform-agnostic files outside of LIBCORO_FEATURE_THREADING

[bjones1]

This implements the suggestion in #170.

[bjones1]

Do you want me to merge in the main branch, or rebase this branch to main? I'm happy to, just let me know your workflow.

Is there anything else to do on this PR?

[jbaldwin]

Generally whatever works best, I usually only squash and rebase into main so it's a singular commit in the final git log.

[bjones1]

Looks like you merged main in successfully -- thanks! Do you need anything else from me on this PR?

I'm excited to get this merged; I think this will get us to successful builds on XCode and MSVC. I'll do some more testing/cleanup after the merge.

[jbaldwin]

I was planning on merging this tonight, but I did take one last look at it and I do think some of the test files can probably be moved outside of the feature flag pretty easily by switching the tests from coro::io_scheduler to coro::thread_pool. Unless if there is another reason you included them inside the feature flag? Would you mind taking a look and letting me know what you think.