2.4.0: Forward compatibility, PHP 8.3 fix

• Fixed PHP 8.3 compatibility in number helper. This is due to the
  change how negative decimals were handled in number_format(). Before,
  they were simply ignored (which is what we relied on), now they're
  used for rounding, see php/php-src#11487
  (@thekid)
• Added forward compatibility with xp-forge/frontend version 5.0, see
  xp-forge/frontend#37
  (@thekid)