Lua API: Worldgen / Terrain formula

[Unarelith]

@KdotJPG To me there could perhaps be three PR stages:

1. Setup lua scripting for per-biome terrain formulas, adding it to the dimension rather than individual biomes
2. Implement biome smoothing, and move it into the biome definitions
3. Revamped biome placement that places+sizes the biomes correctly no matter what the parameters are.

Example of API for terrain formulas:

mod:dimension { -- or later mod:biome
    -- ...
    
    terrain_formula = function(block_pos) {
        noise1 = simplexSmooth(block_pos * 0.0625))
        noise2 = simplexSmooth(block_pos * 0.03125)) * 0.5
        return (1 - abs(noise1)) * (1 - abs(noise2))
    },

    -- ...
}

Note: This would use operator overloads to generate an internal "node graph" of objects, which then will be used in the actual worldgen instead of calling the function each time.

@KdotJPG feel free to add your ideas here.