Make dedicated keymap and mode for eshell-command (Bug#41370)

Make dedicated keymap and mode for eshell-command (Bug#41370)

Otherwise, we end up permanently modifying eshell-mode-map when
running eshell-command.
* lisp/eshell/eshell.el (eshell-command-mode): New mode, with map to
contain the bindings previously set by eshell-return-exits-minibuffer.
(eshell-return-exits-minibuffer): Make into obsolete alias for
eshell-command-mode.
(eshell-command): Use eshell-command-mode instead of
eshell-return-exits-minibuffer.