Factor out other shared Actions to this repo

[dfsnow]

Various repos such as the R packages use a lot of the same shared, custom Actions. Where possible, we should factor out their Actions to a workflow or composite action in this repo.

• Create composite pre-commit action #2
• Refactor test-coverage action for R packages #28
• R package linting (e.g. lint.yaml)
• Build the R package website (e.g. pkgdown.yaml)
• R package build matrix (e.g. R-CMD-check.yaml)