Move region index creation from vcztools

[tomwhite]

We should remove the vcztools index command and have bio2zarr create region indexes instead. (Region indexes are used by vcztools to do efficient region queries.) Then bio2zarr would be concerned only with writing vcz and vcztools only with reading vcz.

This was suggested before (sgkit-dev/vcztools#37 (comment)), but it would be good to move it out of vcztools now before releasing it.

Should index creation be a part of vcf2zarr encode?

[jeromekelleher]

Should index creation be a part of vcf2zarr encode

I think that's the simplest approach all right. It's a very simple operation and we can think about how to make it more generally accessible later on.