Support 'comint-pager' in Eshell

Support 'comint-pager' in Eshell

* lisp/eshell/esh-var.el (eshell-variable-aliases-list): Add "PAGER".
(eshell-var-initialize): Make 'comint-pager' buffer-local and bind it
in subcommands so that we can temporarily set it as necessary.

* test/lisp/eshell/esh-var-tests.el (esh-var-test/pager-var/default)
(esh-var-test/pager-var/set, esh-var-test/pager-var/unset)
(esh-var-test/pager-var/set-locally): New tests.

* doc/misc/eshell.texi (Variables): Document this (bug#63778).

Co-authored-by: Morgan Smith <Morgan.J.Smith@outlook.com>