Support multiple instances of Unity Editor

It's sometimes useful to have multiple instances of the unity editor open so multiple projects can be worked on/referenced simultaneously. For instance, on macOS you can do this by typing `open -n -a Unity` in terminal. This works fine with MonoDevelop and Visual Studio for Mac - Unity just launches two instances of the external editor.

Ideally, Rider would either open the second project as a separate solution in the same Rider process, or spawn a new process. But what happens now (at least on macOS) is that the first Unity project to try to open a file in Rider gets its solution opened, but the other ones appear to just open as individual files.