[Feature] Configurable vanilla structure generation stage

[Astrashh]

Pre-Request Checklist

• I have checked that I am on the latest version of Terra.
• I have searched github for similar features requests, including closed
  ones, and found none.
• I believe this is within the scope of Terra.
• This feature request is for all of Terra, and isn't something that
  should be implemented by a pack or addon.

Feature Description

Add the ability to determine when vanilla structures are generated in the pack.yml stages key.

What Problem Does This Solve?

Allows specifying which features should come before all structure gen (e.g those that are a part of terrain). Without control over this ordering, many features intended to be part of the terrain may cause unintended overlapping with structures.