Split out immutable NormalizedNode implementations

Description

Many yang-data-api users are interested only in the immutable NormalizedNode implementation hosted in yang-data-impl. Split it out to yang-data-spi, so it can be used independently of others.

This also entails dealing with Builders and ImmutableNodes classes. The new artifact should only have ImmutableNodes, which provides a superset of methods from Builders and ImmutableNodes.

Furthermore OrderingAware constructs, like MapNode and LeafListNode should have three basic builder factories:

class ImmutableNodes {
    public static CollectionNodeBuilder<MapEntryNode, ? extends MapNode> mapBuilder(Ordering ordering) {
        return switch (ordering) {
            case SYSTEM -> systemMapBuilder();
            case USER -> userMapBuilder();
        };
    }

    public static CollectionNodeBuilder<MapEntryNode, SystemMapNode> systemMapBuilder() {
        return ImmutableMapNodeBuilder.create();
    }

    public static CollectionNodeBuilder<MapEntryNode, UserMapNode> userMapBuilder() {
        return ImmutableUserMapNodeBuilder.create();
    }
}

Access to individual builders is mediated through NormalizedNode.BuilderFactory and the shapes of builders dictated by NormalizedNode.Builder interface hierarchy.