Resolve reactivity in parse_source_file or delegate to edit_area

See the related planning issue for additional context: #233472 (closed)

1. Additionally reference this suggestion !41920 (comment 412481241) prior to implementation as it may prove ideal.
2. Consider simplification on back of Toast UI's recent update of yaml front matter support. See !41920 (comment 412463080)

The idea here is that we may be able to leverage ToastUI's recent front matter related update such that we don't need to manage two sources (source w/front matter and source without).