Move ImageCore.jl to an extension

[rafaqz]

ImageCore pull in a lot of dependencies that most of the time we don't need.

Can we put it in an extension?

See: rafaqz/Rasters.jl#258 (comment)

[yeesian]

Can we put it in an extension?

I'm open to doing so -- ImageCore became a dependency when we wanted to enable simpler image processing workflows before packages like Rasters.jl existed, and because I didn't (and still don't) know the right way(s) for allowing users to opt out of dependencies.