Revisit core commands location #1212

Currently, it is required to add two dependencies to get core commands like exit and help. As a spring shell user, I was surprised that adding the spring-shell-core dependency to the hello world sample worked fine with my hello command, but I was not able to exit the shell! The exit command was not provided by default and I had to add a separate jar, spring-shell-standard.

This was quite surprising to me to have add another dependency to get "core" commands like exit and help.