Block placement constraints

[Unarelith]

Mods should be able to define constraints for their blocks, for example:

mod:block {
   id = "default:potato",

   -- ...

   constraints = {
      block_whitelist = {
          {{0, 0, -1}, "default:farmland"} -- check that block below is default:farmland
      },
   },

   -- ...
}

It should be possible to define whitelists, blacklists, use offset to check blocks, use groups, and maybe:

sometimes things are more complex, like, "you can place X on block Y, but only if there's no block Z below that"

Checklist:

• Block placement constraints with blacklist/whitelists (final version)
• Allow using groups
• Server-side check (probably in World directly)