Use path to app as part of entrypoint for Express apps

[wch]

For running Express apps, this PR makes it so that setting an environment variable is no longer needed for uvicorn to launch the app; instead, if the app path is:

path/to/app.py

then this sets the entrypoint to

shiny.express.app:path_2f_to_2f_app_2e_py

This will make it possible to deploy on shinyapps.io.

The escaping rule is this:

• For all characters that are not numbers or letters escape them by replacing with _<hex>_, where <hex> is the hexadecimal representation of the character.
• In addition, if the first character is a number, escape it the same way. This is because variable names in Python can't start with a number. For example, 0app_2e_py is not a valid name, and so can't be used as part of an entrypoint, as in shiny.express.app:0app_2e_py.

In my testing, I found that uvicorn actually would accept other characters, like if you run:

uvicorn shiny.express.app:path/to/app.py

However, gunicorn would error if you run:

gunicorn shiny.express.app:path/to/app.py

because in gunicorn tries to parse the part after the : as an ast.Name or ast.Call, and path/to/app.py contains characters that are invalid for either of those types.

I decided to play it safe and make it general by escaping to valid Python variable names.

[jcheng5]

This is impressively pragmatic 🎩