Draft: shell32: Add environment variable to prevent symlinking home folders

This MR has a similar goal !3259, but the difference is that this MR aims to only isolate the XDG user dirs and nothing else. This is entirely possible to do through winecfg, but the problem with using winecfg is that you would have to manually run it on every new wineprefix, and therefore to automate this process without winecfg would require a manual removal of the symlinks. My solution was to create a simple on/off environment variable to disable or enable the creation of the symlinks.