Add stableNull annotation to force tracking mutable fields

[noti0na1]

class A:
  @stableNull var s: String | Null = null
  def getS: String =
    if s == null then s = ""
    s

The annotation will be added as private in scala for now. We plan to make it public in the next major release.

[olhotak]

I think stableNull would be a better name.

[SethTisue]

If I understand correctly, then before becoming public, this would be considered by the SIP committee as part of the explicit-nulls feature.

[olhotak]

LGTM