Provide our own implementation of paragraph navigation in Eshell

Provide our own implementation of paragraph navigation in Eshell

This lets us finally obsolete 'eshell-prompt-regexp', making it
simpler for users to customize their prompts.

* lisp/eshell/em-prompt.el (eshell-prompt-function): Update docstring.
(eshell-prompt-regexp): Make obsolete.
(eshell-prompt-initialize): Remove 'eshell-prompt-regexp' code.
(eshell-prompt-mode-map): Remap paragraph navigation commands to...
(eshell-forward-paragraph, eshell-backward-paragraph): ... these new
commands.