Adopt Ruby example

Ruby tree with a greet package and a stdlib-only solo package.

bazel run @rules_dx//:dx -- init //examples/adopt-ruby/...
bazel run @rules_dx//:dx -- generate //examples/adopt-ruby/...
bazel build //examples/adopt-ruby/...
bazel test //examples/adopt-ruby/...

Both tests pass (greet_spec, solo_test).