Allow scalar iteration in the REPL on 1.9+.

[maleadt]

Thanks to JuliaLang/julia#48400, we can now detect the REPL task and tweak how scalar indexing is treated. Specifically, I think we should allow scalar iteration in the REPL's frontend task, which is only going to be used to render objects (a common source of scalar iteration). Current GPUArrays already tried to do this by checking isinteractive(), but since that may be a false positive, we were still warning about scalar iteration.

Old behavior, with JLArrays' output functions disabled (and show hence triggering scalar iteration):

julia> jl([1])
1-element JLArray{Int64, 1}:
┌ Warning: Performing scalar indexing on task Task (runnable) @0x00007f0e401b4650.
│ Invocation of getindex resulted in scalar indexing of a GPU array.
│ This is typically caused by calling an iterating implementation of a method.
│ Such implementations *do not* execute on the GPU, but very slowly on the CPU,
│ and therefore should be avoided.
│
│ If you want to allow scalar iteration, use `allowscalar` or `@allowscalar`
│ to enable scalar iteration globally or for the operations in question.
└ @ GPUArraysCore ~/Julia/pkg/GPUArrays/lib/GPUArraysCore/src/GPUArraysCore.jl:85
 1

# trigger scalar iteration on a non-REPL task
julia> jl([1])[1]
┌ Warning: Performing scalar indexing on task Task (runnable) @0x00007f10049fc010.
│ Invocation of getindex resulted in scalar indexing of a GPU array.
│ This is typically caused by calling an iterating implementation of a method.
│ Such implementations *do not* execute on the GPU, but very slowly on the CPU,
│ and therefore should be avoided.
│
│ If you want to allow scalar iteration, use `allowscalar` or `@allowscalar`
│ to enable scalar iteration globally or for the operations in question.
└ @ GPUArraysCore ~/Julia/pkg/GPUArrays/lib/GPUArraysCore/src/GPUArraysCore.jl:85
1

# do that again (to show that the warning is only emitted once)
julia> jl([1])[1]
1

New behavior:

julia> jl([1])
1-element JLArray{Int64, 1}:
 1

julia> jl([1])[1]
ERROR: Scalar indexing is disallowed.
Invocation of getindex resulted in scalar indexing of a GPU array.
This is typically caused by calling an iterating implementation of a method.
Such implementations *do not* execute on the GPU, but very slowly on the CPU,
and therefore should be avoided.

If you want to allow scalar iteration, use `allowscalar` or `@allowscalar`
to enable scalar iteration globally or for the operations in question.
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] assertscalar(op::String)
   @ GPUArraysCore ~/Julia/pkg/GPUArrays/lib/GPUArraysCore/src/GPUArraysCore.jl:82
 [3] getindex(xs::JLArray{Int64, 1}, I::Int64)
   @ GPUArrays ~/Julia/pkg/GPUArrays/src/host/indexing.jl:9
 [4] top-level scope
   @ REPL[3]:1

Fixes #446