Add `roc dev` subcommand

[rtfeldman]

This makes roc dev do the "build and then only run if there were no errors" logic which currently the plain roc command does.

The reason to split them out is so that the plain roc command can be reserved for #!/usr/bin/env roc scripts - which need to be in that format (no subcommands allowed, because some env implementations don't support passing through arguments to the specified executable).

Now we can say "use roc dev for normal development" and optimize the plain roc command for the scripting use case (e.g. have it ignore expects, whereas roc dev should use them).