[HED] Editable 'Levels' Description

Support is required to view and edit this field, which is ingested via the events.json sidecar. These changes, along with other dataset-wide tag changes, should also propagate to the downloadable events.json sidecar

Start by locating the code that reads and exposes the HED "Levels" field from the events.json sidecar. Trace how editable dataset-wide tags are saved and how the downloadable sidecar is generated. Done means the field can be viewed and edited, and the updated value propagates to the downloaded events.json sidecar.

Written by the indexing model from the issue text.