Add more remove methods for `ArrayNode`, `ObjectNode` [STEP-3]

[cowtowncoder]

(note: part of https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-3)

Currently there is just one shared removal method for ArrayNode, ObjectNode, declared in ContainerNode:

 public abstract T removeAll();

So let's add some more:

• T removeIf(Predicate<? super JsonNode>)
• T removeNulls(): (short-cut for removeIf(JsonNode::isNull))

We could also consider clear() (similar to Collection), however, given there is already removeAll() let's not add that.

Also considered but not yet added:

• T removeEmpty(): (short-cut for removeIf(JsonNode::isEmpty))