quick fix create value/attribute : automatically add 'variable' annotation when the value is mutated

with this pull request :
```ceylon
class Foo(){
  void bar(){
  	a = 10; // apply quick fix here
  }
}
```
you get
```ceylon
class Foo(){
  shared variable Integer a = 0;
  void bar(){
  	a = 10;
  }
}
```