Stardew Valley: Extract fillers creation in its own file and update item classification post fill

[Jouramie]

What is this fixing or adding?

This PR does two things.

• First, it refactors the fillers creation to be more readable and moves it to its own file, but exposes it through the items module.
• Second, it allows changing items classification once the fill is completed. This allows the prog balancing to do a better job since it will now know that some of the fillers are in fact progression. Based on this conversation: https://discord.com/channels/731205301247803413/1214608557077700720/1343381004357734492

How was this tested?

Run a couple of generation and make sure similar fillers are generated

If this makes graphical changes, please attach screenshots.

N/A

[agilbert1412]

Comments are nitpicks, approved