Investigate using new experimental "Apps" feature of Pkg.jl to complement or replace `create_gap_sh`

[fingolfin]

See https://pkgdocs.julialang.org/v1/apps/ for more.

A bunch of things are still unclear to me about this feature.

But if it is positive, perhaps we should also consider using it for Oscar.jl (to make an oscar app)

[fingolfin]

I asked for examples of package using this feature (for experiments), and these were suggested:

• https://github.com/KristofferC/Rot13.jl -- super minimal & simple
• https://github.com/fredrikekre/Runic.jl -- real world, more complex
• make an ijulia "app" entrypoint JuliaLang/IJulia.jl#1211