support renv.lock ingestion from rig add

[malcolmbarrett]

Closes #294

[gaborcsardi]

Thanks! Have you seen the new rig proj subcommand in the dev version of rig? Ultimately, we'll have

rig proj deploy

to install both R and the packages the project needs, in one go. This currently does not work for renv projects, only for DESCRIPTION files, but we should make it work for renv projects as well.

That said, we could certainly make something like

rig add ./<path-to-renv-lockfile>

work. OTOH

rig add <directory>

may be more ambiguous, but we could require a ./ prefix in this case as well.

The other potential problem is that sometimes it is not possible to tell if an R installation needs updating or not, e.g. on macOS if you are using R-devel or R-next versions. But maybe this is OK and we can give some warnings.

On macOS it could be also annoying the we cannot install multiple patch releases of R, so if two projects needs R x.y.z1 and R x.y.z2, then you'll keep reinstalling R when you switch between them. But that's probably also OK.