Add command to run a packaged Shiny app as a gadget

[gadenbuie]

This could be used to enable Shiny-based gadget apps that could be run as shortcuts. Here's a sketch of a shortcut to run the chattr R app.

    {
        "key": "cmd+k cmd+c",
        "command": "shiny.r.runApp.code",
		"name": "Chat with chattr",
        "args": {
            "langId": "r",
            "code": "chattr::chattr_app()"
        }
    },

The app would start up in a named terminal and the app would open in a simple browser window so that it could be moved around among the editor panes.

We should follow progress in posit-dev/positron#3173, if we can use multi-session support in Positron, that would improve the utility via hooks available in rstudioapi.