Field method refactor for Marc transform

Why these changes are being introduced:
* These updates are required to implement the architecture described
in the following ADR: https://github.com/MITLibraries/transmogrifier/blob/main/docs/adrs/0005-field-methods.md

How this addresses that need:
* Add field methods and corresponding unit tests:
  content_type, contents, contributors, dates,
  edition, holdings, links

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-288