Show node contents in XML format

[dantleech]

This PR adds a node:show command which outputs the XML content of a node (non recursively, not showing binaries).

Syntax highlighting is achieved using the kukulich/fshl package.

This should be exapanded to:

• Provide a node:edit command (export to external editor, reimport)
• Provide alternative formats, e.g. YAML

[dantleech]

Change name to node:dump -- and I wonder if it should incorporate the functionality of session:view:export ? i.e. add a file|output option.

If so then session:import should also be renamed node:import - which makes (more) semantical sense.