Add support for PublishPress Pro

[mcaskill]

Checklist

• [x] I've read the Contributing page.
• [x] An issue does not exists
• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

Description

Adds support for PublishPress Pro plugins which uses Easy Digital Downloads.

How has this been tested?

I'm testing this on a client project that uses Advanced Custom Fields Pro, ACF Extended Pro, and WPML.

Types of changes

• Added class PublishPressPro, based on WpAiPro, for fetching its EDD download URL.
• Added package junaidbhura/publishpress-* to Installer::getDownloadUrl().
• Added references to the README.md.
• Supported packages:
  • junaidbhura/publishpress-authors-pro
  • junaidbhura/publishpress-blocks-pro
  • junaidbhura/publishpress-capabilities-pro
  • junaidbhura/publishpress-checklists-pro
  • junaidbhura/publishpress-permissions-pro
  • junaidbhura/publishpress-planner-pro
  • junaidbhura/publishpress-revisions-pro
  • junaidbhura/publishpress-series-pro