Add `steep setup`

[soutaro]

steep init command just creates Steepfile, but a command that does more for setting up the project would help.

1. Generating Steepfile
2. Setting up rbs collection
3. Adding a bin/steep
4. Running rbs prototype rb
5. Generating RBS definitions of missing gems automatically

🤔 How can we implement the 5?

[Forthoney]

Hi @soutaro. I would like to work on this issue if people have not taken this task.

[ksss]

Regarding points 4 and 5, would orthoses be helpful?