Markdown Support Arrives for Work Items

Introduced general availability of Markdown support for large text fields in Azure DevOps work items. Users can opt-in to use a Markdown editor for individual work items and fields, with a best-effort conversion from existing HTML. This conversion is irreversible. New work items can default to Markdown based on user preference. Markdown formatting can also be applied via the REST API by setting the `/multilineFieldsFormat` for specific fields.