Update decision logic on whether to use yarn to the same way ember-cli does

[kategengler]

Maybe extract a util package from ember-cli? If so, also use for the determination of yarn pnp (#214)

[arcanis]

As mentioned in https://github.com/ember-cli/ember-try/issues/214#issuecomment-437022648, you might be interested in process.versions.pnp to check whether PnP is enabled or not 😃