Disentangle 'shell' and 'syntax' APIs.

The APIs in the org.jnode.shell and org.jnode.shell.syntax packages are mutually dependent. If we are going to put the shell + interpreters into a different project to syntax + help + the command classes, we need to reduce the API coupling. Ideally, different commands should be able to use different syntax mechanisms and still benefit from JNode's syntax aware completion, etcetera.