Prefixed Dompdf library for PublishPress

Prefixed build of dompdf/dompdf and its runtime dependency stack. The Dompdf namespace becomes PublishPress\Dompdf.

The generated package also prefixes php-font-lib, php-svg-lib, Masterminds HTML5, and Sabberworm CSS Parser so no upstream namespaces are registered by PublishPress plugins.

How to update the prefixed library

1. Change the pinned dompdf/dompdf version in require-dev.
2. Set version to the upstream version plus the next fourth digit.
3. Run composer update to rebuild lib/ and the version loader.
4. Run composer test:unit, then composer test:integration.
5. Review and commit the generated code.
6. Create a GitHub release using the four-digit version so Packagist sees the tag.

Then update the plugins that consume publishpress/dompdf-dompdf.