2.0.0: XP 12 compatibility

This second major release upgrades this library to support only the latest PHP 7 version, PHP 7.4, in alignment with XP 12. See xp-framework/rfc#343

• Changed test.verify.Runtime to use PHP_OS_FAMILY constant instead
  of PHP_OS. The former has been available since PHP 7.2
  (@thekid)
• Made this library compatible with XP 12:
  • Dropped support for PHP < 7.4
  • Adopted nullable type syntax, array unpacking
    (@thekid)