Scalafmt v1.6.0-RC4

NOTE. There are known regressions in this v1.6 pre-release (see label), it is recommended to stay on the stable v1.5.1 until all regressions have been fixed.

The IntelliJ plugin has been published in the "nightly" channel instead of the normal "stable" channel.

• #1230 by @gabro Fixes a regression in v1.6 where the new default trailingCommas=never removes a significant parentheses