Add off-task slayer unlock

[TastyPumPum]

Introduce a purchasable unlock to allow killing OSRS slayer-only monsters off task for users who want to farm bosses like Cerb, Sire, Araxxor, etc., without a slayer task.
Keep existing behaviour for slayer boosts so they still require being on a slayer task.
Ensure kill validation respects the new unlock while preserving wilderness and wildy-only restrictions.

• Add OffTaskSlayer to SlayerTaskUnlocksEnum and include a new SlayerRewardsShop entry named Off-task Slayer with slayerPointCost: 5000 and aliases.
• Update newMinionKillCommand to allow monster.slayerOnly kills when slayerUnlocks includes SlayerTaskUnlocksEnum.OffTaskSlayer.
• Keep other checks intact such as wildySlayerOnlyMonsters, wilderness-only rules, and boost/method restrictions.

• I have tested all my changes thoroughly.