Treat escaped newlines in Eshell as the empty string

Treat escaped newlines in Eshell as the empty string

This fixes a regression introduced during Emacs 29's development.

* lisp/eshell/esh-arg.el (eshell-parse-argument): Handle
'eshell-empty-token' as the result of an argument-parsing hook.
(eshell-parse-argument-hook): Document 'eshell-empty-token'.
(eshell-parse-backslash): Return 'eshell-empty-token' when
encountering an escaped newline.

* test/lisp/eshell/eshell-tests.el (eshell-test/escape-nonspecial)
(eshell-test/escape-nonspecial-unicode)
(eshell-test/escape-nonspecial-quoted)
(eshell-test/escape-special-quoted): Move from here...

* test/lisp/eshell/esh-arg-tests.el (esh-arg-test/escape/nonspecial)
(esh-arg-test/escape/nonspecial-unicode)
(esh-arg-test/escape-quoted/nonspecial)
(esh-arg-test/escape-quoted/special): ... to here.
(esh-arg-test/escape/special, esh-arg-test/escape/newline)
(esh-arg-test/escape-quoted/newline): New tests.

* doc/misc/eshell.texi (Arguments): Explain escaping logic in more
detail (bug#59622).