Serialize report-level measures into one or more TMDL files when saving a report using the Power BI enhanced report format (PBIR)

Currently, thin reports saved using the Power BI enhanced report format (PBIR) store the DAX code for report-level measures embedded as JSON strings within a file called "reportExtensions.json", with new line characters and double quote marks needing to be escaped with a backslash.

Report-level measures should instead be defined within one or more TMDL files inside the report's "definition" folder, that extend the base semantic model of the report. That would give report developers a better source control experience when modifying report-level measures, with clean diffs free of escape characters, and the ability to copy and paste DAX code. This is the same experience that semantic model developers already have with the TMDL file format.