Delay Ark startup until debugger is attached

[lionel-]

Branched from #741
Addresses #740

This PR makes it possible to automatically delay the startup of Ark until the debugger is attached to the process:

• This is controlled by a new positron.jupyterAdapter.attachOnStartup setting. Set it to true in your settings.json to enable attach on startup.

  Edit: There is now also positron.jupyterAdapter.sleepOnStartup which can be combined with the above. Useful to attach the debugger and give some time to hit the pause button to enter some commands before resuming startup.

• Attach on startup is supported via a new undocumented argument --startup-notifier-file passed to Ark. It takes the path to a notification file to which the frontend writes when the debugger has been attached. Ark blocks until a change to the file is detected. After that, startup continues as normal. Implemented in Add --delay-startup argument to Ark ark#35.

• The actions of adding this argument and attaching the debugger need to be performed in the start() method of the Jupyter kernel. To support this, Jupyter kernels now optionally accept a new JupyterKernelExtra object containing custom implementations for this sort of extra functionality. This extra object is created and passed in by the Ark registration routine.

[lionel-]

Is it reasonable to add this sort of debugging functionality to the jupyter interface @jmcphers? It's not meant to be a stable part of the API, it just enables a debugging/development convenience while preserving abstraction.

[lionel-]

Kernels that do not support a delayed startup can still provide an attach() method that runs <f5> in parallel to startup. Perhaps we should do that by default when the attachOnStartup option is set. However that can be confusing if the user doesn't realise the debugger and the kernel are run in parallel.