finalize RecommendedTBBVersions format

People use downloaders/scripts to Download TBB. (There is torbrowser-launcher as well as tb-updater and perhaps more.)

The problem is, the RecommendedTBBVersions format is currently undocumented and unfinalized. It changed very often in past and therefore broke these TBB downloaders. This is non-ideal. This is a feature request to finalize the RecommendedTBBVersions format to keep these downloaders functional.