2.0.0: PHP 7.4+, visual refresh

• Changed main font from Overpass to Barlow - its rounded edges
  fit better with the rest of the design
  (@thekid)
• Upgraded XP Compiler & Core - dropping support for PHP 7.0 - 7.3
  (@thekid)
• Merged PR #59: Use Collection::modify() instead of invoking
  "findAndModify"
  (@thekid)