deployApp should fail if the app contains deployApp

[simon-smart88]

Similar to #100 this is a common problem when novice users try and deploy apps and come looking for help on Stack Overflow. There is no reason why you would ever want to do this, so it would seem sensible to prevent it as early as possible. I'm happy to submit a PR to address this and #100 if there is consensus it would be sensible to do so.

[aronatkins]

There are valid reasons to have content which deploys other content. For example, you may have a Shiny application which receives input data, code-generates an R Markdown file, then publishes the result as a new item hosted by Posit Connect.

That code is often going to use a package like pins or connectapi to do that code-driven deployment, but we have also seen rsconnect::deployApp() used in that way.