Call Windows-related code that requires .NET 5/Framework from a .NET 5 helper library that includes stuff like "LTrim".

Doing that should eliminate the need to have the calling app figure out the files and URLs beforehand. If the user isn't on Windows, it'll just autoremove the flags for now until I can figure out a way to support them on Linux and such for those that would need them. This library can be named "libdotdesktop-netcorehelpers".