New user option 'tab-line-tabs-buffer-group-function'

New user option 'tab-line-tabs-buffer-group-function'

* lisp/tab-line.el (tab-line-tabs-buffer-group-function):
Turn defvar into defcustom with the default value
'tab-line-tabs-buffer-group-by-mode'.
(tab-line-tabs-buffer-group-by-mode): New function with body from
'tab-line-tabs-buffer-group-name'.
(tab-line-tabs-buffer-group-by-project): New function.
(tab-line-tabs-buffer-groups): Use fallback name "No group" instead of "All".

(cherry picked from commit 414f8d02c1a361fa780e55fcf0f260fe00a9a62d)