Add tabulated-list-groups and Buffer-menu-group-by (bug#69305)

Add tabulated-list-groups and Buffer-menu-group-by (bug#69305)

* doc/lispref/modes.texi (Tabulated List Mode):
Add defvar tabulated-list-groups.

* lisp/buff-menu.el (Buffer-menu-group-by): New defcustom.
(Buffer-menu-unmark-all-buffers): Use tabulated-list-get-entry
to check whether the current line contains an entry.
(list-buffers-noselect): Enable outline-minor-mode
for tabulated-list-groups.
(list-buffers--refresh): When Buffer-menu-group-by is non-nil,
set tabulated-list-groups.
(Buffer-menu-group-by-mode, Buffer-menu-group-by-root):
New functions.

* lisp/emacs-lisp/tabulated-list.el (tabulated-list-groups):
New buffer-local variable.
(tabulated-list-print-fake-header): Add distinct overlay
property 'fake-header'.
(tabulated-list-header-overlay-p): Filter out overlays that
don't have the property 'fake-header'.
(tabulated-list-print): Use the variable 'tabulated-list-groups'
to sort entries in groups separately.
(tabulated-list-print-entries): New function factored out from
'tabulated-list-print'.

* test/lisp/emacs-lisp/tabulated-list-tests.el (tabulated-list-groups):
New test.