Provide an API that can be used within a notebook to determine if it is running headless

[jonsequitur]

As automation support is added, it might be useful for notebook and extension authors to be able to determine whether a notebook is running interactively or via automation.

It's also possible that a set of kernels could be operating in mixed mode, where some are being driven remotely by others. This might not be a concern if UI-targeting commands (e.g. RequestUserInput) can be routed back to a UI somewhere in the kernel topology.