Add a "Water world biome" option, to allow water colour to be matched

perhaps this could be implemented as a default value if the chunk isn't in the biomestructure?

No files or tests are named. Start by locating the biome structure handling and the code that determines water colour for rendered chunks, then trace how a default biome could apply when a chunk is absent from that structure. Done means the new “Water world biome” option can match the intended water colour without breaking existing biome-based rendering.

Written by the indexing model from the issue text.