Function collectTakenShortcodes

Collect the shortcodes already used by same-type documents in doc's uniqueness scope, excluding doc itself.

• embedded — the owning actor's items of the same type.
• pack — the compendium pack's entries of the same type (read from its index; a build-time lint:packs is the authoritative pack guard).
• world — the world collection (game.items / game.actors) of the same type.