fix: race condition when getting iOS inspector port

[BenSjoberg]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it. "NativeScript debugger has opened inspector socket on port.." is not shown for every simulator when run multiple simulators and livesyncing #3701
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

When running an iOS simulator app, the NativeScript CLI sends an AttachRequest notification to the application right after launch. If the runtime hasn't had time to set up the event handler for this notification yet, it's silently lost, and the CLI never gets the port number for debugging. This causes HMR to not work, and worse, the application auto-restarts after a minute. (The console shows the error "NativeScript debugger was not able to get inspector socket port")

This seems to be a race condition based on how quickly the iOS runtime sets up its event handler vs. how quickly the CLI sends the AttachRequest notification after launching the app. For some reason, I went from not experiencing this issue at all to experiencing it nearly 100% of the time when using the simulator on my Mac. (I haven't tested with an actual device. Also, I'm using the V8 iOS runtime.)

What is the new behavior?

I set up a timer to re-send the AttachRequest notification once every 5 seconds until getDebuggerPort() either returns or throws an error. In my testing, I was able to resolve it by adding a simple one-second delay before sending AttachRequest, but this seems more robust.

Fixes/Implements/Closes #3701. (Seems to be the same issue.)

[NathanWalker]

Thank you @BenSjoberg ! We'll include this with the upcoming 8.1 release.