Combine rterm/rpath settings for different operating systems?

We have separate settings for R paths on different platforms:

• r.rpath.windows, r.rpath.mac, r.rpath.linux
• r.rterm.windows, r.rterm.mac, r.term.linux

But as far as I can tell we don't handle these differently in the code. We check the user's OS, then choose the setting for that OS. So I think we could just have r.rpath and r.rterm. This would simplify things for the user (fewer settings) and for our code.

The downside to this change would be that a lot of users would already have those settings set. I think settings can be automatically migrated - I believe this was done recently when the vscode-jupyter extension was split out from the vscode-python extension. I could look into how they did it and see whether it seems practical. If the user has multiple of those settings set, we could show a message to ask them to choose which one to retain.