Adds support for --install in yarn init

[arcanis]

Summary

Initializing projects using Yarn v2 is fairly annoying when the v1 is the global binary (which it often is), since you need to first run yarn init, then yarn policies set-version, then change the package.json to match what you'd expect if the v2 was initialising the project.

To streamline the process, the --install flag will automatically call yarn policies set-version followed by yarn init.

Test plan

n/a

[buildsize]

File name	Previous Size	New Size	Change
yarn-[version].noarch.rpm	1.18 MB	1.18 MB	191 bytes (0%)
yarn-[version].js	4.86 MB	4.86 MB	1.2 KB (0%)
yarn-legacy-[version].js	5.05 MB	5.05 MB	1.19 KB (0%)
yarn-v[version].tar.gz	1.19 MB	1.19 MB	199 bytes (0%)
yarn_[version]all.deb	869.41 KB	869.65 KB	252 bytes (0%)