Implements yarn init --install

[arcanis]

What's the problem this PR addresses?

I've implemented yarn init --install in yarnpkg/yarn#7723, but it's currently missing in the v2.

More generally, the problem is that starting a new Berry project is fairly annoying since it requires multiple commands:

yarn init -y
yarn policies set-version berry
<edit the package.json>

How did you fix it?

The --install flag forces a Yarn version to be installed in the target directory, and the yarn init command that will run will be provided by the version configured. This makes it very easy to start new projects with any Yarn version you'd want.