Add renv initialization

[ngiangre]

Why?

It may be useful for others wanting to use the app to activate the required packages to load the app.

Other edits

I fixed a gargle options around line 150 in the README as it was part of a gargle lifecycle deprecation warning.

Feel free to not merge this if you don't feel appropriate :)

[MargotBr]

Hello @ngiangre

Thank you very much for your contribution.

I will analyze your PR :)

I had decided not to freeze the development environment with {renv}, so that users would not be in a constrained environment. I've recently had problems using some packages via {renv} with some OS (Windows, especially).

But I'll look into it.

Thanks also for the other changes.

Margot