[#6335] Add ModifyItem advancement for enchanting existing items

[arbron]

Adds a new advancement type that allows for applying an enchantment to an existing item on an actor's sheet. This advancement contains a list of enchantments to apply, and a list of identifiers that are used to find the items to apply those enchantments to.

When configured directly on a class, species, or background, the configuration dialog doesn't include the selection dropdown nor the "Disassociate" button because there is not "Effects" tab visible on those item types.

During the advancement process, each enchantment will be displayed with a list of matching items below it.

Future Improvement

• Allow user control over what items are actually enchanted
• Support rider activities & effects once Move enchantment riders from activity to effect itself #6357 is resolved

Closes #6335

[pgervais70]

What if a matching item is added later?
You would need to reapply the advancement?

[arbron]

What if a matching item is added later? You would need to reapply the advancement?

Yep

[Zanderaf]

Any plans for this to be retroactive in the future like mentioned in the original issue?

[maragondi]

It would be great if an enchantment could automatically apply to the appropriate items when they appear on an actor, and automatically disappear if the item that provides the enchantment is removed.

This would make it possible to create a lot of cool things. For example, I could imagine a homebrew feature that improves a whole group of cold school abilities, or even a full talent system.

I realize this is probably a pretty niche use case, but automatic application would still be extremely convenient.