Worker Kernel Manager

[stefnotch]

With gzuidhof/starboard-python#5 , Starboard-Python is going to run in a web worker. Furthermore, it has been designed with the ability to run more kernels for different cell types in the same web worker.

The kernel manager code is still in the starboard-python repository but should eventually be moved to starboard-notebook. Potential features that could be added to it would be

• Running in a SharedWorker that stays alive during development (useful when the web worker doesn't change, makes reloads more instantaneous)
• Interrupting a kernel (relevant Add pyodide-interrupt package pyodide/pyodide#852 and Interrupting execution pyodide/pyodide#676)
• Restarting a kernel (relevant Restart pyodide and clear user defined variables pyodide/pyodide#703 )
• Access a shared filesystem