Support YAML in web validator

[danielnaab]

Extended Description As a user, I would like to validate my OSCAL document that is in YAML format, so I can use my preferred format.

Acceptance Criteria

• [ ] YAML validation works in the web interface

Definition of Done

• [ ] Acceptance criteria met
• [ ] Unit test coverage of our code > 90% - needs automation story
• [ ] Accessibility tests pass - needs automation story
• [ ] Automated code quality checks pass
• [ ] Security reviewed and reported - check in with Wes on what we could do here
• [ ] Reviewed against plain language guidelines
• [ ] Design QA passed
• [ ] Code must be self-documenting
• [ ] No local tech debt
• [ ] Documentation updated
• [ ] Architectural Decision Record completed as necessary for significant design choices
• [ ] PR reviewed & approved
• [ ] Source code merged