12.0.0: PHP 7.4+, Runner removal

• Merged PR #14: Change from set to use prefix for argument methods
  (@thekid)
• Dropped support for PHP 7.0 - 7.3, step 1 or xp-framework/rfc#343
  (@thekid)
• Merged PR #13: Remove Runner, fold implementation into CmdRunner
  (@thekid)
• Changed code to use new reflection library, see xp-framework/rfc#338
  (@thekid)