Update create new file step from Python walkthrough to use the new command

[luabud]

#18376 (comment)

[luabud]

To verify, make sure you're using the pre-release version of the Python extension. Then open the Python walkthrough (Get Started: Open Walkthrough... command and select the "Get started with Python development" one), click on the create a Python file tile and click the "Create Python File" button. It should create an untitled file with type set to Python to the left of the screen.