fix: Handle "relative" and "proportional" with encumbrance

[Coolthulhu]

Checklist

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• I wrote the PR title in conventional commit format.
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Purpose of change

Fix #3861

Describe the solution

Use assign in a hacky way to have it handle the fields properly.

• Add tests that confirm it works

Describe alternatives you've considered

Revert the armor portions thing - it won't help us much, but it is certain to cause trouble until we rewrite the jsons to the new format.

Testing

• Spawn chitin gauntlets and biosilicified chitin gauntlets
• Compare encumbrance - biosilicified should have 15 encumbrance, while regular should have 10
• Load world with Magiclysm
• Spawn chitin gauntlets and demon chitin gauntlets
• Demon chitin gauntlets should have 9 encumbrance, while regular should have 10

Additional context

[scarf005]

LGTM, but playtest needed @chaosvolt

[chaosvolt]

• Compare encumbrance - biosilicified should have 15 encumbrance, while regular should have 10

Invalid for testing since I manually set encumbrance overrides for stuff that was broken by this bug in vanilla:

  {
    "id": "gauntlets_acidchitin",
    "copy-from": "gauntlets_chitin",
    "looks_like": "gauntlets_chitin",
    "type": "ARMOR",
    "name": { "str": "pair of biosilicified chitin gauntlets", "str_pl": "pairs of biosilicified chitin gauntlets" },
    "description": "Gauntlets crafted from the carefully cleaned and pruned biosilicified exoskeletons of acidic ants.  Acid-resistant and very durable.",
    "material": [ "acidchitin", "leather" ],
    "price_postapoc": "1750 cent",
    "encumbrance": 15,
    "proportional": { "weight": 1.125, "volume": 1.13, "price": 1.25, "warmth": 1.5 },
    "relative": { "bashing": 1, "material_thickness": 1, "environmental_protection": 1 }
  },

Demon chitin items do still have encumbrance modifiers since I forgot about magiclysm though.

[chaosvolt]

1. Compiled and load-tested.
2. Loaded up a save with magiclysm.
3. Compared chitin armor with demon chitin armor, they correctly show different encumbrance now.

As noted, we'll want to reimplement this in vanilla for items we had to work around the broken feature with in the future.