The current test suite against R requires users to install R and multiple packages from CRAN into the global R environment.

Optimally, we would directly install R and all R dependencies available on conda-forge directly from conda-forge into the pyfixest dev environment, which should help developers to run unit tests against R locally + keep the dev's R installation "clean".

I think it would be best to reorganize the unit tests against R to have the following structure:

• tests-against-r-core: tests against packages available via conda-forge: fixest and sandwich. These are directly added to the tests environment as dependencies.
• tests-against-r-extended: tests against R packages not availabe on conda-forge: did2s, ritest, fwildclusterboot, etc.

To do's:

• add the following R dependencies to the tests environment: r-fixest, r-sandwich, r-broom, R itself
• create a new pytest marker against-r-core & decorate all tests that only depend of fixest, sandwich, broom
• create a new pytest marker against-r-extended & decorate all tests that only depend on "other" r packages
• adjust the github actions to install fixest, sandwich, broom from conda
• update the dev guide