Use extensions for GPU packages

[omlins]

This PR works except that the type aliases defined in the extensions cannot be made available to the user of the package (and it needs to be verified if the similar method defined has an effect).

To illustrate that the extensions work in general, the function some_function was added temporarily. It works as shown here:

julia> using CUDA


julia> using CellArrays
[ Info: Precompiling CellArrays [d35fcfd7-7af4-4c67-b1aa-d78070614af4]
[ Info: Precompiling CellArrays_CUDAExt [0f7d8a02-766a-50ac-8bd6-eb945167c0c4]

julia> some_function(CUDA.zeros(2))
2-element CuArray{Float32, 1, CUDA.Mem.DeviceBuffer}:
 1.0
 1.0

GPU unit tests pass (but only because the type aliases were defined directly in the test file):

Test Summary:     | Pass  Total   Time
test_CellArray.jl |  295    295  34.7s
     Testing CellArrays tests passed 

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1a4ed60) 63.56% compared to head (3deb122) 63.56%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   63.56%   63.56%           
=======================================
  Files           3        3           
  Lines         129      129           
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  Hits           82       82           
  Misses         47       47           

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[omlins]

Replaced by #26