* lisp/tab-bar.el: Tab groups can be displayed with tab-bar-format-tabs-groups

* lisp/tab-bar.el: Tab groups can be displayed with tab-bar-format-tabs-groups

* lisp/tab-bar.el (tab-bar-format): Turn defvar into defcustom.
Add :options and force-mode-line-update in :set.
(tab-bar--format-tab): New function refactored from tab-bar-format-tabs.
(tab-bar-format-tabs): Move most of code to tab-bar--format-tab and call it.
(tab-bar-tab-group-format-function): New defcustom.
(tab-bar-tab-group-format-default): New function.
(tab-bar--format-tab-group, tab-bar-format-tabs-groups): New functions.
(tab-bar-format-align-right, tab-bar-format-global): Shorten id.
(tab-bar-change-tab-group): Add refs to tab-bar-format in docstring.