add_bookmark() method #1640

It would be useful to support creating and manipulating Word bookmarks through the public API. I am currently developing a report-generation system in which engineers create Word templates containing bookmarks that act as placeholders. During report generation, these bookmarks are replaced with actual values.

At present, this requires direct manipulation of the document's XML structure. A high-level API for creating, finding, and updating bookmarks would make template-driven document generation significantly easier and more maintainable.