Make Colors.jl a direct dependency

[alyst]

Currently, Colors.jl is a weak dependency, and the user needs using Colors to enable ColorsExt extension, which sole purpose is to define _json_lower(c::Colorant).
However, PlotlyBase.jl already has ColorSchemes.jl as a direct dependency and defines _json_lower(::ColorScheme) in the main module.
But the latter method needs _json_lower(::Colorant) to work.

So, making Colors.jl a direct dependency of PlotlyBase.jl would simplify things a bit. It would not increase the "dependencies burden" of PlotlyBase.jl since Colors.jl is already a direct dependency of ColorSchemes.jl.

Also, it should solve the error

MethodError: Cannot `convert` an object of type JSON.Writer.CompositeTypeWrapper{RGB{Float64}} to an object of type String

which I am getting despite using Colors in my user project.

[hhaensel]

I fully support this PR

(PlotlyBase) pkg> st -m Colors
Project PlotlyBase v0.8.19
Status `~/.julia/dev/PlotlyBase/Manifest.toml`
  [5ae59095] Colors v0.12.11

[alyst]

@sglyon Could you please consider this PR? I've just rebased it to the current master.

[sglyon]

Thank you @alyst