Add tiers of chests

[Matteas-Eden]

User Story As a game designer, I'd like to have different tiers of chests, so that a player has some guarantee of the rarity/type of items that can receive from a given chest.

Acceptance Criteria

• Gold, silver and bronze chest tiles are added
• Gold chests only give the player high level items (Steel Armour, Amethyst/Diamond Rings, Dragon's Bane, Lich Bane)
• Silver chests only give the player low level items (Leather Armour, Steel Sword, Broadsword)
• Bronze chests only give potions (Health Potion, Great Health Potion)
• Different tiers of chests are placed at appropriate places in appropriate levels (i.e. higher level chests in later levels)

Notes

These changes are not intended to modify the Gold/EXP given out by chests currently, but that may be an additional AC.

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Why is this feature needed? Please describe the problem your requested feature wants to solve To give the player more guarantees surrounding the items received from chests.

Describe the solution you'd like At different points in the game, a player encounters differing tiers of chests. Each tier gives out certain items exclusive to that tier.

[Matteas-Eden]

This will need to be adapted to account for the changes in #190.