Z-level split.

Splits each individual Z-level into its own map file.

**Pros**
* Work on different Z-levels will no longer cause merge conflicts.
* Git history size will not grow as rapidly as only individual Z-level files are saved, instead of the entire map.
* Somehow the total map-size went from 3.2MB to 2.86MB. Expected the opposite but the map seem to function fine. Did expect to list this as a con but I won't complain.
* Reduced time needed to save individual Z-levels while editing.

**Cons**
* Map changes on master will for all practical purposes be lost until the split is merged into the branch.
* PRs that include map changes (didn't see any that weren't mine at the time of posting) will have to wait until this PR is either rejected or merged.
* Downstream forks will have to be careful about merging the map tool changes if they stick to the tgstation-named map.