Detect datapacks in worlds and ask whether they should be loaded

Similar to how we prompt for loading a world's resourcepack, we should also add a prompt for datapacks. This would enable loading custom biomes, paintings and banner patterns (and anything that will be data-driven in the future).

Start by finding how Chunky detects a world's resourcepack and presents its loading prompt. Trace the corresponding world-loading path for datapacks, then determine where the new confirmation should appear. Done means worlds with datapacks prompt whether to load them, with behavior analogous to resourcepacks.

Written by the indexing model from the issue text.