Move `real(::Type{<:AbstractGray})` from ColorVectorSpace

[kimikage]

In practice, the method does not have much impact because of the fallback in Base. However, there are some differences as follows:

master w/o ColorVectorSpace

julia> real(Gray{Float32})
Float32

julia> real(Gray24)
ERROR: MethodError: no method matching zero(::Type{Gray24})

julia> real(Gray)
N0f8 (alias for Normed{UInt8, 8})

this PR (or w/ ColorVectorSpace)

julia> real(Gray{Float32})
Float32

julia> real(Gray24)
N0f8 (alias for Normed{UInt8, 8})

julia> real(Gray)
ERROR: MethodError: no method matching zero(::Type{Any})

[codecov[bot]]

Codecov Report

Merging #248 (d44fb14) into master (0c96dbe) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   82.47%   82.49%   +0.02%     
==========================================
  Files           8        8              
  Lines         736      737       +1     
==========================================
+ Hits          607      608       +1     
  Misses        129      129              

Impacted Files	Coverage Δ
src/ColorTypes.jl	100.00% <ø> (ø)
src/conversions.jl	86.81% <100.00%> (+0.14%)	:arrow_up:

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