Add list_commands

[jtpio]

Fixes #4.

Usage

from ipylab import JupyterFrontEnd

app = JupyterFrontEnd()
app.commands.list_commands()

['__internal:context-menu-info',
 'apputils:activate-command-palette',
 'apputils:print',
...
]

[jtpio]

This returns a [] when executed in the same cell:

app = JupyterFrontEnd()
app.commands.list_commands()

This is because the model has not had time to be constructed on the frontend and send the list of commands back to the kernel.

We could add a shell_ready event that can be consumed from Python, so users could await it.