Fix and improve behavior of 'eshell/clear'

Fix and improve behavior of 'eshell/clear'

* lisp/eshell/esh-mode.el (eshell-clear): New function.
(eshell/clear): Fix incorrect behavior, and do the right thing when
'eshell-scroll-show-maximum-output' is nil.
(eshell/clear-scrollback): Call 'eshell/clear'.

* test/lisp/eshell/esh-mode-tests.el
(esh-mode-test/clear/eshell-command)
(esh-mode-test/clear/eshell-command/erase)
(esh-mode-test/clear/emacs-command)
(esh-mode-test/clear/emacs-command/erase): New tests.

* etc/NEWS: Mention the new 'eshell-command' (bug#73722).

(cherry picked from commit 98e24e369a3f6bed95cdf0b32ee03999f5dfb98b)