[ENHANCEMENT] Define `BlockSparseArrays.blocktype(randn(2, 2))`

Define BlockSparseArrays.blocktype for dense arrays. Currently it outputs:

julia> using BlockSparseArrays: BlockSparseArrays

julia> BlockSparseArrays.blocktype(randn(2, 2))
ERROR: MethodError: no method matching blocktype(::Matrix{Float64})
The function `blocktype` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  blocktype(::Type{<:Union{var"#s41", var"#s40"} where {T, N, var"#s41"<:BlockSparseArrays.AbstractBlockSparseArray{T, N}, var"#s40"<:Union{Base.LogicalIndex{T, <:BlockSparseArrays.AbstractBlockSparseArray}, Base.ReinterpretArray{T, N, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s14"}, var"#s14"}} where var"#s14"<:BlockSparseArrays.AbstractBlockSparseArray, Base.ReshapedArray{T, N, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s15"}, var"#s15"}}, SubArray{<:Any, <:Any, var"#s15"}, var"#s15"}} where var"#s15"<:BlockSparseArrays.AbstractBlockSparseArray, SubArray{T, N, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, Base.ReshapedArray{<:Any, <:Any, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, var"#s16"}} where var"#s16"<:BlockSparseArrays.AbstractBlockSparseArray, LinearAlgebra.Adjoint{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Diagonal{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.LowerTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Symmetric{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Transpose{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Tridiagonal{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UnitLowerTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UnitUpperTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UpperTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, PermutedDimsArray{T, N, <:Any, <:Any, <:BlockSparseArrays.AbstractBlockSparseArray}}}})
   @ BlockSparseArrays ~/.julia/dev/BlockSparseArrays/src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl:88
  blocktype(::Union{BlockSparseArrays.AbstractBlockSparseArray{T, N}, Base.LogicalIndex{T, <:BlockSparseArrays.AbstractBlockSparseArray}, Base.ReinterpretArray{T, N, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s14"}, var"#s14"}} where var"#s14"<:BlockSparseArrays.AbstractBlockSparseArray, Base.ReshapedArray{T, N, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s15"}, var"#s15"}}, SubArray{<:Any, <:Any, var"#s15"}, var"#s15"}} where var"#s15"<:BlockSparseArrays.AbstractBlockSparseArray, SubArray{T, N, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, Base.ReshapedArray{<:Any, <:Any, <:Union{Base.ReinterpretArray{<:Any, <:Any, <:Any, <:Union{SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, SubArray{<:Any, <:Any, var"#s16"}, var"#s16"}}, var"#s16"}} where var"#s16"<:BlockSparseArrays.AbstractBlockSparseArray, LinearAlgebra.Adjoint{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Diagonal{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.LowerTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Symmetric{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Transpose{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.Tridiagonal{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UnitLowerTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UnitUpperTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, LinearAlgebra.UpperTriangular{T, <:BlockSparseArrays.AbstractBlockSparseArray{T, N}}, PermutedDimsArray{T, N, <:Any, <:Any, <:BlockSparseArrays.AbstractBlockSparseArray}} where {T, N})
   @ BlockSparseArrays ~/.julia/dev/BlockSparseArrays/src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl:87

Stacktrace:
 [1] top-level scope
   @ REPL[34]:1

That will be useful in generic code. Like BlockArrays.jl, we should in general use a convention that non-blocked arrays act like block sparse arrays with a single stored block.

Start by reproducing the BlockSparseArrays.blocktype(randn(2, 2)) error and read the existing methods in src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl, especially around lines 87–88. Compare the current convention with BlockArrays.jl as suggested in the issue, then verify that dense arrays are treated as having one stored block and that generic calls work.