Add new Tramp method "flatpak"

Add new Tramp method "flatpak"

* doc/misc/tramp.texi (Inline methods): Add flatpak.

* etc/NEWS: Add new Tramp method "flatpak".  Fix typos.

* lisp/net/tramp-container.el (tramp-flatpak-program): New defcustom.
(tramp-flatpak-method): New defconst.
(tramp-flatpak--completion-function): New defun.  Set it for "flatpak".
(tramp-methods) <flatpak>: Add.
(tramp-container-connection-local-default-flatpak-variables):
New defconst.  Set respective connection-local variables.