Add new ZoneScopeManager for web

[obecny]

If you have any suggestions, thoughts, things I should be aware of when implementing the new ZoneScopeManager that will support async operations please share it, thank you.

[obecny]

cc @mayurkale22 @draffensperger

[draffensperger]

I think we could base this loosely on the OpenCensus Web tracer that uses Zone.js.

Also, I think the core functionality of the scope manager should bring in Zone.js as a peerDependency, and call the package something like opentelemetry-scope-zone-peer-dep. The advantage of that is that for Angular apps that bring in Zone.js already, there are no issues around version skew or problems around bringing in the Zone.js library twice.

That said, for non-Angular apps (the majority!), we could have a more convenient package opentelemetry-scope-zone that just composes opentelemetry-scope-zone-peer-dep with the Zone.js package itself. We could clearly document which package to use when based on whether your app already includes the Zone.js dependency.

We did this for OpenCensus Web, see https://github.com/census-instrumentation/opencensus-web#using-as-npm-dependency-in-existing-js-build in the second bullet point.