Missing specialized findnext/findprev methods for BitArray

[nalimilan]

We currently provide optimized methods for findnext/prev(testf::Function, B::BitArray, start::Integer), but not for findnext/prev(testf::Function, B::BitArray, start::CartesianIndex). There are two problems:

• The lack of optimized BitArray methods for CartesianIndex means that the slow AbstractArray fallback is used, in particular by findfirst and findlast.
• The methods for Integer are inconsistent with Array/AbstractArray, for which we don't support passing a linear index (except for vectors). That's not a big deal since these additional BitArray methods don't conflict with the AbstractArray ones (and it would make sense to add them for AbstractArray for when you want a linear index result).

See also #30102.