[New rule] prefer-abstract-field

[fzyzcjy]

Please describe what the rule should do:

abstract class A {
  double get a;
  set a(double value);
}

can be written as following:

abstract class A {
  abstract double a;
}

The linter should warn for the first case (so users can write the second case)

If your rule is inspired by other please provide link to it:

What category of rule is this? (place an "X" next to just one item)

• [ ] Warns about a potential error (problem)
• [x] Suggests an alternate way of doing something (suggestion)
• [ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about (it will be better if you can provide both good and bad examples):

Are you willing to submit a pull request to implement this rule?