Draft: Add a basic work items code generator

Closes #60 (moved)

TODO:

• Build into the REPL
• Allow selection of a new code generator
• Transform widget-based filters (such as weight)
• Account for transformation of field names into strings
• Account for transformation of field names into widgets
• Prevent the use of queries without group/project scope

Additional changes demonstrated in this MR:

• Returning errors from Code Generators
• Restricting use of fields to a known set