* lisp/tab-bar.el: 'C-x t N' bound to tab-new-to supports a negative argument
* lisp/tab-bar.el (tab-bar-new-tab-to): Negative TO-INDEX counts tabs from the end of the tab bar. (tab-bar-new-tab): Fix docstring to add reference to 'tab-bar-new-tab-to'. (tab-prefix-map): Bind "N" to tab-new-to.