implement `{renv}`

[cjyetman]

This issue keeps coming up again and again in different contexts....

1. How do users know what packages are needed?
2. How do users install the packages they need?
3. What if PACTA requires a specific version of a package that is not the current release version?
4. etc.

{renv} manages the dependent packages in your repo and ensures that any user will be using the correct set and the correct versions of all those dependent packages.

https://rstudio.github.io/renv