add an assist to convert a constructor parameter to a field (w/ initializer)

[pq]

From flutter-insiders feedback:

• When you extract a widget class, or add a new constructor parameter to an existing widget class via quick fix, what you end up in is this:
  Constructor({String newVar});

However, below is where you want to get to and you have to edit the above manually to get to this below:
Constructor({required this.newVar});
String newVar;

It would be nice to have one or more refactor options that take us from the second one to the first one, such as "assign to a new field", "make required", etc.

[bwilkerson]

Note that there is work to consider for the existing quick fix(es), assuming we can identify which ones. Minimally, it should make the parameter required when in a null-safe library. But if there is anything that would indicate that a field is what the user is likely to want we should create the field directly without requiring a second step.