Feature: #84203 - Unify form setup YAML loading

See forge#84203


Form setup files of the "form" extension now make use of the TYPO3 core YAML file loader. This allows using the known TYPO3 core features:

  • import of other YAML files via imports directive
  • replacement of %placeholders%


Form setups can now be structured more freely by splitting logical parts into separate files.