Dynamic Row Manager

Write createRowManager() that RETURNS an object managing rows ({ id, label }): - add(label) — appends a row with a unique **increasing** id starting at 1 and returns the id - remove(id) — removes that row; returns true if it existed, false otherwise - list() — returns the rows in insertion order - move(id, toIndex) — moves the row to toIndex in the list; returns true on success, false when the id is unknown or toIndex is out of range