Support structure block keys with a list of items

[solonovamax]

Support using structure block keys which contain a list of items

For example:

{
    "keys":   {
        "S":      "minecraft:sea_lantern",
        "p":      [
            "minecraft:prismarine",
            "minecraft:prismarine_bricks",
            "minecraft:dark_prismarine"
        ],
        "d":      [
            "minecraft:dark_prismarine",
            "minecraft:prismarine",
            "minecraft:prismarine_bricks"
        ],
        "anchor": "minecraft:conduit"
    },
    ...
}

Each key is either a string or an array of strings, both of which are parsed by BlockArgumentParser.blockOrTag().

I'm probably going to PR a change later that does this. (it's a not-too invasive change in StructureTemplate.parse, as well as a few additional utility classes just to make the code cleaner and avoid the anonymous classes.)

[solonovamax]

An alternative syntax could be:

{
    "keys":   {
        "S":      "minecraft:sea_lantern",
        "p":      "minecraft:prismarine,minecraft:prismarine_bricks,minecraft:dark_prismarine",
        "d":      "minecraft:dark_prismarine,minecraft:prismarine,minecraft:prismarine_bricks",
        "anchor": "minecraft:conduit"
    },
    ...
}

and then splitting it on ,. Though, care would need to be taken to not split it on commas in the nbt data.

This one is more similar to patchouli's syntax.