Ensure navigating by paragraphs in Eshell stops at prompts and paragraphs

Ensure navigating by paragraphs in Eshell stops at prompts and paragraphs

The previous implementation in 6ae2b74ed20 only stopped at prompts,
which isn't the right behavior (bug#61545).

* lisp/eshell/em-prompt.el (eshell-forward-paragraph)
(eshell-backward-paragraph): Reimplement to handle prompts and
paragraphs (the latter by calling the original 'forward-paragraph').

* test/lisp/eshell/em-prompt-tests.el
(em-prompt-test/next-previous-prompt/multiline): Rename.
(em-prompt-test/forward-backward-paragraph-1): New function.
(em-prompt-test/forward-backward-paragraph)
(em-prompt-test/forward-backward-paragraph/multiline): New tests.