Configurable sub-level assembly height (#938)

[tatoyoda600]

Adds 2 config values, implementing suggestions 2 & 3 from #938:

• SUB_LEVEL_FIXED_START_HEIGHT: If SUB_LEVEL_START_POS is set to FIXED, then this is the height (Default value of 128 matches the current approach)
• SUB_LEVEL_START_POS: Determines the approach to use for determining the height (Y value) of the first block in a newly assembled sub-level (FIXED, DYNAMIC, MIDPOINT)
  • FIXED: Uses the height specified by SUB_LEVEL_FIXED_START_HEIGHT (Essentially the current approach)
  • DYNAMIC: Uses the world height of the position the block is in (Spawn a sub-level at Y=73, the block is placed at Y=73 in the sub-level)
  • MIDPOINT: Calculates the midpoint between the min and max build limits, then uses that as the height

Defaults are 128 and FIXED, to match the current approach exactly, with all the customization being exclusively opt-in.

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With dynamic assembly height, a 1-block sub-level created at Y=87 has its sole block located at Y=87 within the sub-level

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[tatoyoda600]

Implemented suggestion 1 from #938, but it kinda sucks, so not including in this PR (https://github.com/tatoyoda600/sable/tree/automatic-height-adjustment)
It has some uses and would be great if it could be implemented well, but I don't know how feasible (or worth it) it would be to actually polish it up to a point where it would be truly usable.

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It does help with the issue I was having in #938 though, as soon as the Rope Pulley reaches -64 in the sub-level, the sub-level's contents are shifted up to give it more downwards space to expand, letting my Rope Pulley reach real deep.