[FEATURE] @Singular Map with Immutable Collection

[KyRobbins]

Currently you can use @Singular on a Map to produce an immutable Map with convenient methods on the builder for adding a single entry. If you were to do a Map<?, Collection<?>>, this also works, offering a method to add a collection as a single entry for the builder, but what about adding a single item to a possibly existing collection, and making that collection immutable?

There are other ways it could be done, you could track a Collection<?> as a separate element and add it to a builder only once it is completed, but I think it would be nice to maintain the builder as the primary mechanism

Example:

@Builder
public class Context {
  @Singular
  private final Map<String, List<Constraint>> groupConstraints;
}
//...
private void parseNode(Context.ContextBuilder builder, Node node) {
  //...
  node.getConstraints().stream().forEach(constraint -> builder.addGroupConstraint(constraint.getGroup(), constraint));
  //...
}

Possible Sample Builder Code:

public Context.ContextBuilder groupConstraint(String key, List<Constraint> constraints) {
  if (this.groupConstraints$key == null) {
    this.groupConstraints$key = new ArrayList();
    this.groupConstraints$value = new ArrayList();
  }

  this.groupConstraints$key.add(key);
  this.groupConstraints$value.add(constraints);
}

public Context.ContextBuilder groupConstraint(String key, Constraint constraint) {
  if (this.groupConstraints$key == null) {
    this.groupConstraints$key = new ArrayList();
    this.groupConstraints$value = new ArrayList();
  }

  int lastIndex = this.groupConstraints$key.lastIndexOf(key);

  if (lastIndex > -1) {
    this.groupConstraints$value.get(lastIndex).add(constraint);
  } else {
    this.groupConstraints$key.add(key);
    this.groupConstraints$value.add(constraints);
  }
}