List variables can now extend config items

[Paul-Ferrell]

• List variables can now be used to extend config items.
• Variable lists cannot be extended in this way.

Code review checklist:

• Code is generally sensical and well commented
• Variable/function names all telegraph their purpose and contents
• Functions/classes have useful doc strings
• Function arguments are typed
• Added/modified unit tests to cover changes.
• New features have documentation added to the docs.
• New features and backwards compatibility breaks are noted in the RELEASE.md

[tygoetsch]

As mentioned in the meeting this morning, I think additional documentation of the functions and their behavior would be welcome. Up to you to decide if the current documentation is appropriate though.