asdf has a very useful configuration setting called
`legacy_version_file`. When it is set to `yes`, asdf will read the
version files used by other version managers (e.g. `.ruby-version`) in
addition to asdf's own `.tool-versions` file. This makes the migration
to asdf much smoother, as many projects only specify these 'legacy'
dotfiles for versioning.
Documentation: https://asdf-vm.com/#/core-configuration?id=homeasdfrc
Closes https://github.com/thoughtbot/dotfiles/issues/654
