Use specified dimension separator for region index

[tomwhite]

I noticed that I didn't set the dimension separator when doing #290.

I also noticed that contig_id, contig_length, filter_id, and sample_id don't have a dimension separator set - which OK as they are one-dimensional - but slightly inconsistent as there are other fields that are one-dimensional that do have it set, like variant_id. Should we just set it everywhere?

[coveralls]

coverage: 98.91%. remained the same
when pulling ae1a046 on tomwhite:dim-separator-fix
into b9340c4 on sgkit-dev:main.

[jeromekelleher]

Should we just set it everywhere?

I don't really mind. I set the dimension separator to / as this seemed the more scalable option for the large 2D fields and followed the defaults in v3 (making upgrades to format=3 stores later more straightforward, potentially).