subversion: add meta.mainProgram

pkgs/applications/version-management/subversion/default.nix

···
       description = "A version control system intended to be a compelling replacement for CVS in the open source community";
       license = licenses.asl20;
       homepage = "https://subversion.apache.org/";
+      mainProgram = "svn";
       maintainers = with maintainers; [ eelco lovek323 ];
       platforms = platforms.linux ++ platforms.darwin;
     };