Split structures out of biomes

[BoySanic]

This PR splits out structures and hashing from biomes.zig into structures.zig and src/utils.zig respectively.

This change should be done for the following reasons:

1. It cleans up Biome-independent structure tables #2129. A lot.
2. Splitting it out from biomes.zig into another file makes it a bit cleaner once we add the changes from Biome-independent structure tables #2129 that introduces the StructureTable struct for example. It's technically feasible to leave it all in biomes.zig, splitting it out made the most sense to me at the time for organization purposes.

[nekonesse]

does this allows addons to add structures to preexisting biomes? 🥺

[BoySanic]

does this allows addons to add structures to preexisting biomes? 🥺

With #2129 yes. Not by itself though.

[IntegratedQuantum]

Seems like I totally missed this, please rebase and I'll review this again.

[BoySanic]

I'll get this rebased so we can push it forward.