A unified check_package() function

[wlandau]

Currently, the package has a check_versions() function to check the version compliance of all packages in R-multiverse. The users of the package will mostly be maintainers of specific R-multiverse packages, so I think it would be much more convenient to have a unified check_package() function which queries the overall status of an individual package.

The workflow at https://github.com/r-multiverse/checks/blob/main/.github/workflows/issues.yaml checks packages for compliance. For each package with issues, a JSON file is written to issues/ folder in https://github.com/r-multiverse/checks for that package. Currently the only type of issue checked is version compliance. When R-universe gains a check API, we will add those too. But in any case, check_package() can simply query the issues/PACKAGE_NAME file and write informative output for the user.