Minor rework of shopkeeper_item_group

[andrei8l]

Summary

None

Purpose of change

shopkeeper_item_group currently blocks restocking from using item groups if their conditions are not met. strict parameter prevents buy-back if conditions are not met after restocking.

This means that if the player meets the conditions right after a restock (by ex. turning in a mission or picking the right dialogue option), they'll have to wait until the next restock before they can buy the new items.

I want to change it to:

strict blocks restocking from using item groups if their conditions are not met. Conditions on their own only block selling (ie. npc::wants_to_sell())

This will make the gated items available right away once conditions are met. Item groups that aren't supposed to be visible for some reason (like the Hub 01 UHMWPE armor pieces) can be gated by a strict condition.

Split off from #57952

Describe the solution

Implement

Describe alternatives you've considered

Using a shopkeeper_sell_blacklist like the shopkeeper_blacklist I'm introducing in #57952: seems unnecessary and gross

Testing

Unit test.

Additional context

This will have a real use case in #58052

I-am-Erk I'd like to get your opinion since you originally asked for the shopkeeper_item_group feature
dseguin also yours please since you wrote the original implementation

[andrei8l]

Thanks for merging.