Add option to ignore class member implementations

[Desuuuu]

Motivation

At the moment, abstract class members that have an implementation are never detected as unused because the implementation itself is returned in the references for the abstract class member.

export abstract class AbstractClass {
  public abstract used(): void;
  public abstract unused(): void;
}

export class ExtendedClass extends AbstractClass {
  public used(): void { }
  public unused(): void { }
}

import { AbstractClass, ExtendedClass } from './definitions';

const instance: AbstractClass = new ExtendedClass();

instance.used();

In this example, neither unused method would be detected as unused.

Changes

This PR adds the ignoreClassMemberImplementations option as well as the --ignore-class-member-implementations CLI flag.

When enabled, implementations are ignored when looking for references of class members, allowing for the removal of unused class members which are never directly referenced.

In the previous example, only AbstractClass.unused would be detected as unused at first. If it is removed, then ExtendedClass.unused would be detected as unused.