Add `capture()` API to capture replay from buffered events

[billyvg]

This public API should function very similarly to what we do currently for error-based replays.

This method would only be available in the “on error” replay mode. It performs the necessary steps to create a new replay based on the buffered replay events and then switch the recording mode to be “session”-based so that we can continue capturing afterwards. This would be used by our default “on error” handler, as well as a public API to allow users to capture replays outside of error events (e.g. on button click).

[billyvg]

As an update, instead of creating a new api capture(), we overloaded flush() to accomplish the same thing.