Support file change watching other than in app.py location

[travis-leith]

The shiny command line arg --reload-dir allows the user to specify additional directories to watch for file changes.
It would be great if this is a setting in the extension, so that the run button in vscode could make use of it.

[gadenbuie]

This might actually be better handled in the Shiny package itself, possibly through the use of a configuration file, like a shiny.toml or similar, that contains settings like these.

The problem is that it'd be hard to expose a configuration for a command line arg like --reload-dir in this extension because that value is likely to be different between apps.

[travis-leith]

I actually opened this in the shiny package originally, but closed it before opening here. I have no reopened the original issue so the proper location can be discussed.