Implement a general input fontification mechanism for comint modes

Implement a general input fontification mechanism for comint modes

* lisp/comint.el
(comint-indent-input-line):
(comint-indent-input-line-default):
(comint-indent-input-region):
(comint-indent-input-region-default): New functions that implement a
general mechanism for input indentation through an indirect buffer in
comint derived major modes.
* lisp/shell.el (shell-mode): Set up input indentation according to
sh-mode (bug#51940).