fix: allow postfix setters under language.postfixOps

[KacperFKorban]

Allow for postfix operators to be followed by assigns.
This enables the definition and use of the following syntax (more precisely the parsing of the >_= method as a postfix operator + assign):

val v = new Vector(1, 2, 3)
println(v) // prints <1, 2, 3>
v<1> = 10 // assign 10 to element at index 1
println(v) // prints <1, 10, 3>
println(v<1>) // prints: value at 1 is 10

// Definition of Vector:

class Vector(values: Int*) {
  val data = values.toArray
  class Getter(i: Int) {
    def `>_=`(x: Int) =
      data(i) = x
    def > : Int =
      data(i)
  }
  def < (i:Int) = new Getter(i)
  override def toString = data.mkString("<", ", ", ">")
}

[SethTisue]

my understanding is that language.postfixOps is legacy stuff that's only still in the language as a compatibility/migration aid. so should we be increasing its power? even in Scala 2 it's been considered legacy for some years now and almost nobody uses it anymore

[KacperFKorban]

@SethTisue I think so, but this is something that is allowed in Scala 2, so even for a legacy feature it would be nice to have feature parity. (And here the cost is non-existent).

[SethTisue]

Oh, I didn't understand this just aligns with existing Scala 2 behavior. Sure, no objection then.