Automatically upgrade pythonExperimental to python

[qubitron]

All debug configs that use pythonExperimental need to be upgraded to python, all users of old debug configs will suddenly start failing with the message:

Failed to find debugger extension info for type:pythonExperimental

Can we have a task that runs on startup that automatically scans launch.json, looks for the presence of the string pythonExperimental, if detected we prompt a banner that says

Your launch.json file needs to be updated to change the "pythonExperimental" debug 
configurations to use the "python" debugger type, otherwise Python debugging may 
not work. Would you like to automatically update your launch.json file now?

[ Yes, update launch.json ] [ No, I will do it later ]

If click yes it:

• Rewrites all instances of "pythonExperimental" to "python"
• Rewrites all instances of Python Experimental: to Python:

This should be run on every startup of the Python extension, the banner does not go away if the user selects No, they will see it the next time they load the extension.