(texinfo-mode-syntax-table): Add \n as comment-end.

(texinfo-mode-syntax-table): Add \n as comment-end.
(texinfo-font-lock-syntactic-keywords): New var.
(texinfo-font-lock-keywords): Remove comment regexp.
(texinfo-insert-block): New function.
(texinfo-mode-map): Add C-c C-o -> texinfo-insert-block.
(texinfo-mode): Use define-derived-mode.
Tweak comment-start-skip and font-lock-defaults.
Use regexp-opt for outline-regexp.
(texinfo-environments): New var.
(texinfo-environment-regexp): Use regexp-opt and texinfo-environments.