Use the check-licenses script

[pento]

We should automatically check licenses on PRs.

Running check-licenses on the repo finds a couple of issues:

$ npx wp-scripts check-licenses --dev
 ERROR  Module xmldom has an incompatible license 'LGPL'.
$ npx wp-scripts check-licenses --prod --gpl2
 ERROR  Module detect-libc has an incompatible license 'Apache-2.0'.

xmldom is an easy fix, we just need to add LGPL to @wordpress/wp-scripts.

detect-libc is a little trickier, as it's ultimately included by chokidar, which we do use. nsfw is an interesting alternative to chokidar, it's also used by VS Code.