Folder To Solution Folder
To download and install the extension to Visual Studio 2015/2017/2019: https://marketplace.visualstudio.com/items?itemName=CeciliaWiren-CeciliaSHARP.FolderToSolutionFolder
Create a Solution Folder from a selected folder also including the files in that selected folder. Will not move the files. Access through the context menu for the solution in the solution explorer right below Add->New Solution Folder.