Setup/Enable GitHub Dependabot for public repos

[atc0005]

• https://help.github.com/en/github/administering-a-repository/keeping-your-dependencies-updated-automatically
  • https://help.github.com/en/github/administering-a-repository/enabling-and-disabling-version-updates
  • https://help.github.com/en/github/administering-a-repository/configuration-options-for-dependency-updates

Context: I manually track dependency releases and/or use Visual Studio Code's Go plugin's support for go.mod files to determine when I need to pull in package updates. Dependabot sounds like it would really help out with ensuring I don't miss out of any updates that might be security related.