Add default Drupal gitattributes

[dafeder]

Follow-up to #4592. Replaces #4589.

We should ideally use the default Drupal gitattributes file to enforce things like proper line endings.

Unfortunately at the moment we have a few files that do contain CLRF line endings. This means if the default gitattributes file is applied the repo can get into a broken state where you can't switch branches because line endings don't match.

This issue is to remind us to come back to this, maybe as part of a 4.x release or switching from packagist to drupal.org where we go ahead and fix all those files in one fell swoop.