[POC] Generate node types command

This is just a quick sketch of a command to generate CND files (just namespaces and names, no properties / children definitions).

Obviously there is quite a large scope here, but just to follow on from <what was the issue where we discussed this?>

Next steps:

- [ ] Add configuration option to automatically assign node types to documents
- [ ] Provide node type migrate command (?)
We should ask for confirmation -- later on, if we add property and child item support we could also show a diff as @dbu suggested