Consider R-universe for additional testing and early exposure

[gregorgorjanc]

The idea is that R-universe gives you additional testing and a bit more rapid turnover, exposure, binaries for the platforms than CRAN, but CRAN is stil the ultimate "publication" channel.

Here are instructions https://docs.r-universe.dev/publish/set-up.html

If this goes ahead, then:

• remove the R-universe GitHub action (.github/workflows/r-universe-check.yaml) - this action is rigid (on purpose) but we have our R package in a sub-folder and I could not see how to tweak that
• add r-universe badge usethis::use_r_universe_badge()