Update launch.json schema to add "python" and remove "pythonPath"

[int19h]

Now that we no longer have to worry about debug configs being compatible with both ptvsd and debugpy, there's no reason to use the legacy spelling and semantics for the property. The schema should be updated:

1. Remove "pythonPath" entirely.

2. Add "python", and reflect the fact that it can be either a simple string, or an array of strings (to specify command-line arguments for the Python interpreter itself).

[int19h]

#13218 is now intended as the one and only way of supplying arguments to the interpreter, so "python": [...] shouldn't be exposed in the extension - only scalar string values.

(debugpy will still support the array syntax for backwards compatibility)