RFC: api to retrieve current extension status

[david-shortman]

autoPause prevents the redux devtools extension from doing tracking work.

But this doesn't prevent tools that integrate with the devtools from doing work to prepare and send some state to the devtools.

The devtools should expose some API that allows an integrating tool to query the status of the extension. Then those tools can avoid executing unnecessary code when the extension is closed.

This is relevant in a PR open for NgRx: ngrx/platform#2941 (comment)

I'm willing to work on a PR here, but am not sure what the API should be.

For instance, I think it would be simplest to expose the property directly under __REDUX_DEVTOOLS_EXTENSION__, named something like isExtensionOpen.