feat(balance): allow mills to hold more food than smoking racks

[chaosvolt]

Checklist

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

I spent ages gathering the materials to make half a dozen wind mills, a grand army of giants whose mighty arms would be a match for the strongest of Spanish knights, only to find they only hold as much corn as a smoking rack that only takes 8 rocks and 16 planks to make.

Suggested buffing up the capacity and was told to go full send it and make it 100 liters in capacity:

Describe the solution

1. In iexamine.cpp, added definition MAX_FOOD_VOLUME_MILLING to sm_rack namespace (which is also used by mills), set to 100 liters.
2. In ther same file, changed mill_activate and iexamine::quern_examine to use MAX_FOOD_VOLUME_MILLING instead of MAX_FOOD_VOLUME, thus bumping capacity from 20 to 100 liters.

Describe alternatives you've considered

1. Only increasing capacity to 50 liters like my very first initial idea.
2. Sanity-checking corn because why is a single unit .75 liters jfc also that one farm variant spawns with SO GODDAMN MUCH CORN AAAAA

Testing

1. Compiled and load-tested.
2. Spawned in a windmill, confirmed it says I can fit 100 liters in it.
3. Confirmed smoking racks still show 20 liters.

Additional context

[ZChris13]

ye I know exactly which corn farm you're talking about lol

[chaosvolt]

I really oughta nerf that one somehow, hmmst. :>