Add items to steel mill

[snipercup]

Summary

SUMMARY: Content "Adds items to steel mill"

Purpose of change

helps #34307

Describe the solution

Adds items to the steel mill map special.

• Undid my earlier PR Add items to office rooms in steel mill #34335, it was not an effective method for adding items. I used the palette instead.

• Changed the letters of some furniture to get the right items on them. Counters in the office should have different items then counters in the mill. I did not change any furniture, the steel mill is the same as it was except now there are items.

• Found some useful item groups and added 2 of my own to get as close to the idea of the steel mill author as possible (Adds steel mill location #33383). As far as I can tell there are no iron ores and steel ingots so I just added what I thought was relevant.

• It still needs monsters but that will be in another PR (not necessarily by me).

Describe alternatives you've considered

Leaving as is

Additional context

With increased item spawn for demonstration:

Actual spawn chances:

[kevingranade]

This pull request has been mentioned on Cataclysm: Dark Days Ahead. There might be relevant details there:

https://discourse.cataclysmdda.org/t/i-found-a-steel-mill-severely-dissapointed/21557/3