From November 28th, 2020 to November 28th, 2020, 100% user give 4-star rating. for Salesforce Sharing Button chrome extension.
Seems to work as described. It jumps you into the Classic Salesforce view of the Sharing page where you can select the desired sharing options. Once saved you'll have to manually go back to Lightning via the Lightning hyperlink, if desired. Easy enough work around until Salesforce comes up with an actual Share button in Lightning.