Default to radian console if rterm.preferRadian is true

[BerriJ]

This PR will close #1347

It adds a new option rterm.preferRadian which defaults to false.

If false nothing changes. If rterm.preferRadian is true and the user did not specify a path manually (r.rterm.linux for example). Then vscode-R looks for radian on the PATH and uses it. If radian is not found, it falls back to R.

I would be really happy if this could get merged. I'm also happy to adjust as needed. I deliberately set the default to false so merging this will not change how vscode-R behaves until the user explicitly sets that setting to true. As discussed in the PR, using radian from the PATH can be very helpful in environments where the path to the radian binary is not static.

Thank you so much for this project. I'm a daily user since many years.

Best, BerriJ