I ran through some directions from others, including the very helpful post from SqlVariant, but I had issues locating the correct keys. For my Windows 10 machine, running SSMS 2016, I found the registry keys related to the object explorer located in a different path.
I found matches for read committed/uncommitted string at:
Running the following powershell command:
get-itemproperty -path 'Registry::HKCU\SOFTWARE\Microsoft\VisualStudio\14.0\SSDT\SQLEditorUserSettings' ' select SetTransactionIsolationLevel ' format-list