Add functionality to load one or more region files

Instead of only being able to load a whole world, it would be useful to simply be able to load one or more region files. Many other mapping software support doing this.

The issue does not name files, tests, or entry points. Start by locating the existing whole-world loading flow and how Minecraft region files are discovered and parsed. Done means the application can load a selected single region file or multiple region files, rather than requiring an entire world.

Written by the indexing model from the issue text.