Disallow selecting a directory for the R executable

[rillig]

I just installed the R plugin and was happy that you already did all the work that comes with such a plugin. Of course, as users are, I am lazy and don't read UI text. Therefore, when I had to select an R interpreter, I chose the base directory of the R installation, which was wrong.

This triggered an exception in IntelliJ:

`java.lang.Throwable: Cannot run program "C:\Users\rillig\Program Files\R-3.6.1": CreateProcess error=5, Zugriff verweigert

In the Settings > Languages > R > R Interpreter dialog, it should only be possible to select an executable file, not a directory.

[holgerbrandl]

Great suggestion.