Add "a ?:= 2" support: should be expanded to "a = a ?: 2"

This would allow the very common useful idiom found in Ruby:

variable ||= value

Currently we have to write code like this in Groovy:

def myMap = [:]

if (condition) (myMap[someLongNameObject.getKeyId()] = myMap[someLongNameObject.getKeyId()] ?: []) << someNewValue

Also, if getKeyId() is costly or shouldn't get called more than once for some reason, we would have to write it like:

def myMap = [:]
if (condition) {
  def key = someLongNameObject.getKeyId()
  (myMap[key] = myMap[key] ?: []) << someNewValue
}

When it could be simplified as this:

if (condition) (myMap[someLongNameObject.getKeyId()] ?:= []) << someNewValue

is related to

GROOVY-5306 Add "a ?= 2" support: should be expanded to "a = a == null ? 2 : a"

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