Add BlockStateMeta#clearBlockState

[Machine-Maker]

Resolves #10159

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An alternative would be to make BlockStateMeta#setBlockState nullable. I don't know if we want to make that change, especially since the interface is already setup so the getter is nonnull and there is a has boolean method.

[tal5]

Not sure how big of a PR do you want to make this/maybe split it up into multiple, but as mentioned in the original issue there's a few other places with a similar problem:

• ArmorStandMeta's values are all boolean (i.e. can't be cleared)
• AxolotlBucketMeta's variant can't be cleared
• CompassMeta's LodestoneTracked is a boolean and will always remain in NBT, etc.

Either way thank you very much for adding this!

[electronicboy]

All those 3 boil down into the fundamental flaw of ItemMeta, which is why the property API is coming to be a thing

[Machine-Maker]

Yeah, I agree with @electronicboy here. Those point to specific properties that can be reset. The property API is the way to handle that. Other similar clear methods pertain to whole groups of data, like PotionMeta#clearCustomEffects, FireworkMeta#clearEffects, and more, all exist and I think this falls into that category.