Follow Scala 2.13's lead and make postfixOps syntax an error (not just a warning) unless the feature is explicitly enabled

[smarter]

See scala/scala#6831

[smarter]

Also, assuming support for ancient sbt is not required, per scala/scala#7319

No, we only support sbt 1, and the sbt bridge is in this repository anyway (https://github.com/lampepfl/dotty/tree/master/sbt-bridge)

[SethTisue]

or perhaps just don't support it at all?

[robstoll]

I work on this but need some input -- i.a. if I shall enable the feature for dotty itself or rewrite code. See #8388 for the other questions