ci: update codecov action from r-lib

[kelly-sovacool]

Changes

workflow from test-coverage example here: https://github.com/r-lib/actions/tree/929c772977a3a13c8733b363bf5a2f685c25dd91/examples

Issues

Resolves #36

PR Checklist

(~Strikethrough~ any points that are not applicable.)

• [x] This comment contains a description of changes with justifications, with any relevant issues linked.
• ~[ ] Write unit tests for any new features, bug fixes, or other code changes.~
• ~[ ] Update the docs if there are any API changes (roxygen2 comments, vignettes, readme, etc.).~
• ~[ ] Update NEWS.md with a short description of any user-facing changes and reference the PR number. Follow the style described in https://style.tidyverse.org/news.html~
• [x] Run devtools::check() locally and fix all notes, warnings, and errors.

[codecov[bot]]

Welcome to Codecov :tada:

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered :open_umbrella: