Small bark audit - fixing bark resource density

[Karol1223]

Summary

None

Purpose of change

While working on #75311 I noticed our bark resources have bad densities. That didn't make me feel very warm and fuzzy inside, so I chose violence.

Describe the solution

• Added a new bark abstract, since all of our bark items shared all fields but the ID, name, and description. Willowbark and alder bark also had barter value, but I chose to throw that out the window, since I can't exactly see anyone paying for that when they can just go and tear it off the tree themselves
• The bark abstract has all non-unique fields the same as old alder bark did, with the exception of barter price which is set to 0 like how tanbark had it
• Made the 4 vanilla bark resource items copy from the abstract, removed the 3 breaking density tests from the bad density list
• Renamed all of them to piece of [bark] for nicer pluralization. I fucking love pluralization

Describe alternatives you've considered

Testing

Additional context