Diff - Add Ludwig integration

For this to be reviewed and accepted, please review https://git.drupalcode.org/project/diff#contribution-guidelines. Since this is a decidedly old contribution, it likely needs a reroll/rebase. Additionally, we'll need an MR opened.

From the linked page:

If you are using Composer to manage your Drupal project you don't need Ludwig, and you shouldn't use it!

Non-composer installations are not something I think we need to support.