feat(#3638310): add drush vale:sync to download the styles .vale.ini names

Adds drush vale:sync, which runs vale sync with the site's binary and configuration so the packages that .vale.ini names are downloaded into its StylesPath, then reports how many rules Vale now loads. It exits with 1, with Vale's reason, when Vale is not set up or the sync fails. The README gets an install step for it, and kernel tests cover success, a failed sync, a missing binary and a sync killed before Vale answers.