Add biomes

Add e.g. a separate, much larger scale noise map for generating biomes (e.g. plains, woods, ocean, desert, a mix (i.e. lakes, rivers, etc., like the current world generator)).

The issue names no files or tests; start by locating the current world generator and its noise-map implementation. Determine how terrain types are currently selected, then use the requested biome variety and larger-scale map as the completion criteria.

Written by the indexing model from the issue text.