Quicktags Change Output

There is an option in Quicktags for a new page. It inserts <!–nextpage–> into the post text. Somewhere along the lines, PHP searches for <!–nextpage–> and inserts a page break there.

Where can I find that document? Instead of <!–nextpage–> I would like it to say <!–begin_new_chapter–>, and changing this just in the quicktags.js is not sufficient.