edit method
Obtain a zipper starting at the root of this node. The zipper can be used
to edit the node; call ZipperLocation.commit to obtain a new node with
the edits applied.
Implementation
OrgZipper edit() => ZipperLocation.root(
sectionP: (obj) => obj is OrgParentNode,
node: this,
getChildren: (obj) => obj.children,
makeSection: (node, children) => node.fromChildren(children),
);