Sbb randomized block replacements

[IntegratedQuantum]

The current way to do this is rather tedious and inefficient:

Instead it would be nice to define replacements with a chance.
At runtime this could be implement as a palette filled with tagged unions, where the default case would contain a single block, and the other case would contain an alias table. At runtime this would then require just a single branch.

requires #1412

[Argmaster]

Ok, so isn't this just a more limited version of this: #1412

Child structures blueprints are already sampled from an alias table, so my understanding of this request is that you don't want to create the SBBs to sample from. If you were to just build the structure out of stone, instead of building it out of child blocks, you could use #1412 to randomly replace some percentage of that stone with ore, no?

[IntegratedQuantum]

#1412 only defines fixed replacement, e.g.

.substitute = .{
	.{.old = "cubyz:birch_log", .new = "cubyz:oak_log"},
}

this issue is an extension of #1412, that would allow definining randomized replacements, possibly defined like this

.substitute = .{
	.{.old = "cubyz:stone", .new = .{
		.{.id = "cubyz:stone", .chance = 0.99},
		.{.id = "cubyz:coal_ore", .chance = 0.01},
	}},
}

In the engine this substitution palette would be stored as a list of tagged unions as explained above.