Enable CI for dso-r

[grst]

Close #1.

Adds the following CI services

• automatically run roxygen and pre-commit
• run tests (R CMD check)
• on release, check that the git tag matches the version specified in DESCRIPTION and build a tarball with R CMD build

I don't think we need a separate docs page (pkgdown) for this repo for now, I'd rather include the R-specific documentation in the main docs (sphinx) as well.

[grst]

@DSchreyer, R CMD check currently fails because of a WARNING related to "S3 generic/method consistency"

Could you look into that please?

There's also a NOTE about

 checking whether the namespace can be loaded with stated dependencies ... NOTE
  Warning: no function found corresponding to methods exports from ‘dso’ for: ‘show’

that sounds relevant, but I really don't know if it really matters.

[grst]

btw, due to some github limitations, CI does not run after a pre-commit autoformat has been pushed.

To avoid this setup pre-commit locally (pre-commit install).

If it happens nonetheless, you can push an empty commit to trigger CI:

git commit --allow-empty -m "trigger CI"

[DSchreyer]

LGTM. Tested and works as expected