[WIP] Make Makie a mono repository

[SimonDanisch]

We want to make Makie a mono repository for all related packages:
This needs to get merged first: MakieOrg/Makie.jl#1020

[SimonDanisch]

A sanity check from #35965 (comment):

using TOML
cd("dev/Makie")
packages = ["CairoMakie", "GLMakie", "MakieCore", "WGLMakie"]
for pkg in packages
    versions = TOML.parsefile(joinpath(first(DEPOT_PATH), "registries",
                              "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
            false
        end
    end
    println(pkg, ": ", found ? "all versions found!" : "ney")
end

CairoMakie: all versions found!
GLMakie: all versions found!
MakieCore: all versions found!
WGLMakie: all versions found!

[SimonDanisch]

@DilumAluthge The Makie monorepo PR got merged, so this is ready :)
I tried adding this branch of the registry locally as the only registry and everything seems to work fine, so I think this should all work?