Add assist for additional parameters to be named parameters instead of required parameters

[jwren]

From Flutter IJ issue here, flutter/flutter-intellij#6868:

Currently when I create a class with a final parameter, I have a hint from the IDE to add that field into the constructor. Currently it creates as a simple parameter, such as MyClass(this.myField), and I can also change it to a normal parameter, where it goes into the initialization list.

Since Flutter uses lots of named parameters, it will be very helpful if we could have the option to also have that field being added into the constructor as a named parameter.

[bwilkerson]

We still don't have a direct path to create a named parameter, but we do now have an experimental refactoring that will convert a positional parameter into a named parameter.