conditionally use SpecialFunctions through weak deps or Requires

[t-bltg]

Fix #184.
Fix #182.
Fix last part of JuliaPlots/UnicodePlots.jl#291.

This is a proposal to make all the SpecialFunctions related functions (which are clearly piracy) optional using weak dependencies on julia >= 1.9 and Requires on older releases.

This would be (slightly) breaking so I guess we should bump ColorVectorSpace to 0.10.0 if this PR is merged, since you'd have to use:

julia> using SpecialFunctions
julia> using ColorVectorSpace

to recover all the functionalities on current master.

Note that lfact is renamed to logfactorial since lfact was deprecated in SpecialFunctions.

I followed the guidelines in https://pkgdocs.julialang.org/dev/creating-packages/#Conditional-loading-of-code-in-packages-(Extensions) and https://docs.julialang.org/en/v1.10-dev/manual/code-loading/#man-extensions.

This PR was written for ColorSchemes, and pkg> test ColorSchemes passes locally since ColorSchemes does not depend on the SpecialFunctions functionalities defined in ColorVectorSpace.

Latency improvements:

PR

julia> @time @time_imports using ColorVectorSpace
      1.0 ms  Statistics
     66.4 ms  FixedPointNumbers
    100.7 ms  ColorTypes
      0.4 ms  TensorCore
    319.3 ms  ColorVectorSpace
  0.613890 seconds (553.44 k allocations: 39.516 MiB, 17.56% gc time, 16.28% compilation time)

master

julia> @time @time_imports using ColorVectorSpace
      1.5 ms  Statistics
     85.7 ms  FixedPointNumbers
    124.7 ms  ColorTypes
      8.0 ms  IrrationalConstants
      0.2 ms  Compat
    105.7 ms  ChainRulesCore
      1.4 ms  DocStringExtensions
      0.4 ms  LogExpFunctions
      0.3 ms  OpenLibm_jll
     23.2 ms  Preferences
      0.3 ms  JLLWrappers
      7.0 ms  OpenSpecFun_jll 90.69% compilation time
     22.2 ms  SpecialFunctions
      0.3 ms  TensorCore
    203.7 ms  ColorVectorSpace
  0.642129 seconds (628.22 k allocations: 43.283 MiB, 5.46% compilation time)

Ping @timholy or @kimikage or @johnnychen94 for review (primary contributors to this repo).

[codecov]

Codecov Report

Base: 86.41% // Head: 91.44% // Increases project coverage by +5.03% 🎉

Coverage data is based on head (a48c42c) compared to base (8ec1d35).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   86.41%   91.44%   +5.03%     
==========================================
  Files           2        3       +1     
  Lines         265      269       +4     
==========================================
+ Hits          229      246      +17     
+ Misses         36       23      -13     

Impacted Files	Coverage Δ
ext/SpecialFunctionsExt.jl	100.00% <100.00%> (ø)
src/ColorVectorSpace.jl	90.87% <100.00%> (ø)
src/precompile.jl	100.00% <0.00%> (+100.00%)	⬆️

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[timholy]

Deep review comments 😆 below. I think this is a good idea. We need to make a breaking release soon anyway because we are way behind schedule on https://github.com/JuliaGraphics/ColorVectorSpace.jl#abs-and-abs2.

[timholy]

Thanks so much, @t-bltg!