Added dimension padding and other things

[jacobsjo]

The main reason for this PR is the change to dimension padding made my mojang in 24w44a by fixing MC-272062. In the process, I also changed a few other things:

• Dimension padding now affects the start piece. This leads to different structure placement, requiring porting to deepslate (before it only affected the other jigsaw pieces, which isn't part of deepslate)
  • This is a breaking api change because of the addition of levelHeight to the GenerationContext. This is necessary since this depends on the world height set in the dimension_type, and not the noise_settings.
• StructureSet.getStructureInChunk now returns an object of structure id and starting position. This allows me to display the structure y position on my map. This is also a breaking change
• Fixes:
  • getBaseHeight had the heightmaps flipped
  • the oldBlendedNoise density function used the wrong seeding
  • fixed endIsland density function