Use the 'field' property to navigate through Eshell prompts

Use the 'field' property to navigate through Eshell prompts

* lisp/eshell/esh-mode.el (eshell-skip-prompt-function): Make
obsolete.

* lisp/eshell/em-prompt.el (eshell-prompt-regexp): Update docstring.
(eshell-prompt-initialize): Don't set 'eshell-skip-prompt-function'.
(eshell-next-prompt): Search for the 'field' property set to 'prompt'
to find the next prompt.
(eshell-previous-prompt): Move 'forward-line' call into
'eshell-next-prompt'.
(eshell-forward-matching-input, eshell-backward-matching-input):
Reimplement on top of 'eshell-next-prompt'.
(eshell-skip-prompt): Make obsolete.

* test/lisp/eshell/em-prompt-tests.el
(em-prompt-test/next-previous-prompt): New test.