Make `captureRenderTree` API public

A while back, we added the `captureRenderTree` API as a more stable (but still private/intimate) API for the Ember Inspector to use. It's fairly well thought out though it didn't really go through the RFC process and did not really consider any use cases outside of the inspector. As of now, the status of the API is that it's officially private/intimate, though since the Ember Inspector uses it it's not going to just go away completely without replacement. It's not a huge priority, but if someone would like to use this API, then let's discuss that here and evolve it towards its final form through the RFC process.