Implement set_server()

[jonocarroll]

This package sets the server to the default public server (in onLoad) if no env var is set, but if the user does set one manually later, it isn't obvious how to "reset" that. Given that we have ntfy_server(), perhaps a way to set that means the user doesn't need to use Sys.setenv() so much.

ntfy_set_server <- function(server = "https://ntfy.sh") {
   Sys.setenv(NTFY_SERVER = server)
}