Calculate generationArea coordinates from existing regions file names.

[HydrolienF]

We can find some coordinates that's include all the area to generate by seaching minX, minZ, maxX, maxZ from the region file names.

An issue can be that it might generate more than the selected area if x or z are not 512 muliplier. That's probably does not mater much, because it will only be void chunk as if the chunck where generate by a player near the border.

It would allow users not to configure that part of the config.