✨ Improvements to CI and package structure

[jsnel]

Is your feature request related to a problem? Please describe.

Drawing inspiration from:

• How to write your own R package and publish it on CRAN
• https://blog.r-hub.io/2022/09/12/r-dependency/
• https://cran.r-project.org/web/packages/lintr/vignettes/lintr.html