☁ Fix pre-commit config and files, add code check in GitHub actions

[philipp-baumann]

addresses #49 , still work in progress until review requested

[philipp-baumann]

@ThomasKnecht here is a nice little article with a short outline of styling and linting capabilities in R via the pre-commit framework. https://datawookie.dev/blog/2022/09/enforcing-style-in-an-r-project/

[philipp-baumann]

PR status

• [x] Tweak lintr and styler config for pre-commit

• [x] fix all current files so they pass all checks

• [x] Add to README how to locally install pre-commit hooks

• [x] Add actions yaml template for style and linter checks: for pushing on main branch or merge request (use https://pre-commit.ci/lite.html )

[philipp-baumann]

Ready for your review @ThomasKnecht :rocket: