Scala 3 also allows the empty argument list, so maybe it's not worth cleaning that up in Scala 2?

TIL. I'll try to understand the Scala 3 feature well enough to know if that is plain wrong.

Edit: haven't gotten around to that, but for now I did allow just the new syntax

new T() {} // looks like Java, and arguably they are the parens to the anon class

Should be a lint warning.

Before committing to this PR, what I really wanted to do is try backporting trait params. That way, I'll understand how trait params work (until I forget again).