Add new package regex-posix-clib

[andreasabel]

Checklist:

• Meaningful commit message, eg add my-cool-package (please not mention build-constraints.yml)

• At least 30 minutes have passed since uploading to Hackage

• On your own machine, you have successfully run the following command (find verify-package in the root of this repo):

  ./verify-package $package # or $package-$version
  

[alexeyzab]

Thanks!