Fix external IO loop thead interaction and add function to Base.Experimental to facilitate it's use. Also add a test.

[gbaraldi]

While looking at https://github.com/JuliaLang/julia/issues/55525 I found that the implementation wasn't working correctly. I added it to Base.Experimental so people don't need to handroll their own and am also testing a version of what the issue was hitting.

@vtjnash Is the wakeup of thread 0 for libuv only or do I need to wake both the IO thread and thread 0 always?