add toplevel [workspace] and use projects for Dependabot

[t-bltg]

Per https://discourse.julialang.org/t/psa-github-dependabot-now-supports-julia/134997, and https://pkgdocs.julialang.org/v1/toml-files/#The-[workspace]-section.

[BeastyBlacksmith]

How do I run the tests of say PlotsBase now?

I tried

(Plots) pkg> activate PlotsBase/
  Activating project at `~/.julia/dev/Plots/PlotsBase`

(Plots/PlotsBase) pkg> test
┌ Warning: The project dependencies or compat requirements have changed since the manifest was last resolved.
│ It is recommended to `Pkg.resolve()` or consider `Pkg.update()` if necessary.
└ @ Pkg.API ~/.asdf/installs/julia/1.12.1/share/julia/stdlib/v1.12/Pkg/src/API.jl:1227
ERROR: `LaTeXStrings` is a direct dependency, but does not appear in the manifest. If you intend `LaTeXStrings` to be a direct dependency, run `Pkg.resolve()` to populate the manifest. Otherwise, remove `LaTeXStrings` with `Pkg.rm("LaTeXStrings")`. Finally, run `Pkg.instantiate()` again.

(Plots/PlotsBase) pkg> up
    Updating registry at `~/.julia/registries/General.toml`
ERROR: empty intersection between Plots@0.0.0 and project compatibility 2

[BeastyBlacksmith]

Might have been my Julia version. Woks on 1.12.4

[t-bltg]

(Plots.jl) pkg> test PlotsBase, yes probably supported only on 1.12+, or maybe old Manifest.tomls interfering.