Deprecate 'shiny static' in favor of 'shinylive export'

[wch]

The shinylive package is now on PyPI:

• https://pypi.org/project/shinylive/
• https://github.com/rstudio/py-shinylive

This PR disables shiny static and tells the user to install shinylive and use shinylive export.

When you run shiny static, this is what it now shows:

$ shiny static myapp site
The functionality from `shiny static` has been moved to the shinylive package.
Please install shinylive and use `shinylive export` instead of `shiny static`:

  shiny static-assets remove
  pip install shinylive
  shinylive export APPDIR DESTDIR

[jcheng5]

LGTM, pending final subcommand name

[wch]

I changed the command to shinylive export.