Refactor Zarr.jl into ZarrCore (minimal pure Julia implementation) and Zarr.jl could load all the extra packages.

[felixcremer]

The ZarrCore package would be for power users that know exactly which package extensions they need for their codecs and Zarr.jl would stay for normal users.
The ZarrCore package should live as a subpackage in the Zarr.jl repository.

[asinghvi17]

Let's do this after v3 lands? Otherwise it will be a mess