Track current working directory in each kernel

[jmcphers]

Kallichore currently reports the current working directory of the kernel via the API, but that information is wrong if the working directory has changed since the kernel was started.

There are a few ways we could try to fix this:

• try to get the working directory from the process itself using system APIs
• spy on the messages on the UI comm that are sent to Positron to update its UI
• ask the kernel what its working directory is by using the UI comm ourselves or via some other dedicated means