Add ability for modules to specify new base items without having to duplicate Items compendium

[arbron]

Currently, base items (such as from DND5E.weaponIds) can only be fetched from the compendium listed in DND5E.sourcePacks.ITEMS. This means that any module that wants to add a new base weapon for example, has to completely duplicate all of the items referenced by id in config into their own compendium and change DND5E.sourcePacks to reference their new pack. This is a lot of work and makes it impossible for two different modules to modify the base item types.

I propose a change to how these base item ID lists are handled to allow two different formats: the current format of simply the ID (e.g. "Gflnp29aEv5Lc1ZM") which would cause the item to be looked up in the DND5E.sourcePacks.ITEMS compendium, and a new format that specifies the pack name and ID for the lookup (e.g. "myModule.new-items.Gflnp29aEv5Lc1ZM").

This change should make it easy for module makers to introduce new base items.

[arbron]

Originally in GitLab by @aaclayton

This is a good suggestion, but im not comfortable with adding scope to %"D&D5E 1.4.0 - Armor Class Automation" at the moment. We need to consider that milestone as basically closed at this point.