Monorepo : bring back PlotThemes to v1

[t-bltg]

Description

Add PlotThemes to v1 monorepo, inspired from #4419.

Integrate PlotThemes into Plots repo using git filter-repo:

set -e
set -x
{
  # $ sudo apt install git-filter-repo

  # clone all repos (vanilla)
  if [ ! -d PlotThemes.jl ]; then
    git clone https://github.com/JuliaPlots/PlotThemes.jl
  fi
  if [ ! -d Plots.jl ]; then
    git clone https://github.com/JuliaPlots/Plots.jl
  fi

  PlotThemes=$PWD/PlotThemes.jl
  Plots=$PWD/Plots.jl

  # create a dedicated branch for this PR
  (cd $Plots; git checkout master; git log -2; git checkout -b monorepo-plotthemes)

  # process `PlotThemes`
  (
    cd $PlotThemes
    git filter-repo --to-subdirectory-filter PlotThemes
  )
  (
    cd $Plots
    git remote add PlotThemes $PlotThemes
    git fetch $PlotThemes master
    git merge --allow-unrelated-histories --no-edit FETCH_HEAD
    git remote remove PlotThemes
  )

  exit
}

Sanity check, using https://github.com/JuliaRegistries/General/blob/master/CONTRIBUTING.md#appendix-checking-if-a-repository-contains-all-registered-versions-of-a-package :

using TOML

general = joinpath(first(DEPOT_PATH), "registries", "General")

cd("Plots.jl") do
  for pkg ∈ ("RecipesBase", "RecipesPipeline", "PlotThemes", "Plots")
    versions = TOML.parsefile(joinpath(general, pkg[1:1], pkg, "Versions.toml"))

    found = all(sort(collect(keys(versions)))) do version
      tree_sha = versions[version]["git-tree-sha1"]
      try
        readchomp(`git rev-parse -q --verify "$(tree_sha)^{tree}"`)
        return true
      catch
        return false
      end
    end
    println(pkg, ": ", found ? "all versions found !" : "ney")
  end
end

PlotThemes: v0.3.0 found
PlotThemes: v0.4.0 found
PlotThemes: v1.0.0 found
PlotThemes: v1.0.1 found
PlotThemes: v1.0.2 found
PlotThemes: v1.0.3 found
PlotThemes: v2.0.0 found
PlotThemes: v2.0.1 found
PlotThemes: v3.0.0 found
PlotThemes: v3.1.0 found
PlotThemes: v3.2.0 found
PlotThemes: v3.3.0 found

DON'T SQUASH AND MERGE !, just MERGE !

[t-bltg]

@BeastyBlacksmith, can you archive https://github.com/JuliaPlots/PlotThemes.jl ?

I have transferred all issues and created a PR in the General registry : JuliaRegistries/General#146798.

Thanks.