Add .skip() and .up() method to forEachDescendant

[dsherret]

Right now, forEachDescendant looks like this:

node.forEachDescendant((node, stop) => {
    console.log(node.getText());
    if (someCondition)
        stop();
});

It would be nice to add a .skip() function that skips looking at the descendants of only the current node and an up function that does the same, but also skips looking at the next siblings:

node.forEachDescendant((node, traversal) => {
    console.log(node.getText());
    if (someCondition)
        traversal.stop();
    else if (otherCondition)
        traversal.skip(); // skip looking at the descendants
    else if (otherCondition2)
        traversal.up(); // skip and go back up the tree to the parent's next sibling
});

Inspired by: https://github.com/jamiebuilds/babel-handbook/blob/master/translations/en/plugin-handbook.md#stopping-traversal