as seen in pmmp/BedrockBlockUpgradeSchema@ebd768e5b2, this enables use of newFlattenedName in more places (by allowing the flattened values to be transformed before building the new ID), as well as reducing the number of remappedStates in general by compacting stuff which was partially transformed like color silver -> light_gray.
this significantly reduces the size of schemas when state remaps are used (see pmmp/BedrockBlockUpgradeSchema@85b83b360e).
in addition, this will likely offer a substantial performance and memory saving when walls get flattened, which will eventually happen.
since PHP emits empty JSON objects as arrays, this makes it pretty annoying to work with the schemas in other languages. However, nullability is something most languages understand pretty easily.
This should continue to support old schemas.
this reduces the footprint of RuntimeBlockMapping by a further 1 MB, as well as simplifying various parts of the code, and solidifying the immutability guarantee of BlockStateData.
this spectactularly stupid bug could also have been addressed using a [^\/] in place of the . in the pattern, but I thought it made sense to make it impossible for this to happen again by making sure the regex only sees the file name.
fixes#5129
