Make this repo a Julia package installable with the package manager

I got:
```
(@v1.5) pkg> add https://github.com/h-Klok/StatsWithJuliaBook
    Cloning git-repo `https://github.com/h-Klok/StatsWithJuliaBook`
   Updating git-repo `https://github.com/h-Klok/StatsWithJuliaBook`
ERROR: expected a `name` entry in project file at `/var/folders/rj/njz8jnv11637flz6vgnb6bz00000gp/T/jl_mQRFNN/Project.toml
```