return result from context.next()

[Rich-Harris]

Returns the result of any transformations in child visitors when calling next():

const node = {
  type: 'Foo',
  bar: {
    type: 'Bar',
    transformed: false
  }
};

walk(root, null, {
  Foo(node, context) {
    const result = context.next();
    console.log(result.bar.transformed); // true
  },
  Bar(node) {
    return { ...node, transformed: true };
  }
});

If no transformations occurred, returns undefined.