feat: implement _server.yml standard

[JosiahParry]

This PR adds an internal launch_server() function that satisfies the requirements of the _server.yml standard.

I went ahead and followed the same argument and configuration precedence in the nanonext pr where HOST & PORT env var are respected over everything, followed by the config in _server.yml and lastly the defaults.

I have an issue on precedence open that may be nice to nail down formally.

I based the _server.yml structure on the runApp() arguments and chose only the ones that I suspect would be relevant for a "production" environment.

engine: shiny
options:
    host: "127.0.0.1"
    port: null
    app_dir: null
    quiet: false