Clarify `dev` vs `server` in command descriptions

[cllns]

It's hard to condense the distinction down to just a handful of words, so feel free to tweak.

I also wonder if we should combine the commands somehow, since the distinction here between dev and server isn't intuitive, though I understand how they came to be, of course.

Maybe just hanami dev does the following:

• Does Procfile.dev exist?
  • Yes
    • Does it only have this in it: web: bundle exec hanami server?
      • Yes: Start Rack::Server
      • No: Run foreman
  • No: Start Rack::Server

And then add a flag like hanami dev --ruby to just start Rack::Server if that's needed