Add drush vale:sync to download the styles .vale.ini names

Problem/Motivation

A site that changes its .vale.ini has to run vale sync by hand to download the styles it names, from the directory the site's configuration resolves against. Nothing in the module helps with that, and the install steps leave it to the reader to find the right binary, configuration and working directory.

Proposed resolution

Add drush vale:sync. It runs vale sync with the site's binary and configuration, so it downloads the packages into that configuration's StylesPath, then reports how many rules Vale now loads. It exits with 1 when Vale is not set up or the sync fails, with Vale's own reason. A sync may take five minutes, because a download takes longer than a lint.

The README gets an install step for it, after the step that points settings.php at .vale.ini.

Remaining tasks

Review.