Created by: lukekarrys
This removes the --use-npm flag and adds the --use-yarn flag. It also removes the previous behavior of checking for the existence of yarn as the signal to use it as the package manager. The new behavior will be to use npm by default and to use yarn with the --use-yarn flag.
The goal of this PR is to remove the check on whether yarn is present in the user's path as the signal to use yarn.
This PR is implemented on top of #11304, since it changed some of the same files and was easier to get tests passing on top of that. I will rework this based on the outcome of that PR. For now, here's the differing commits in this PR: https://github.com/lukekarrys/create-react-app/compare/lk/dev-npm-7...lk/use-npm