refactor(eshell): eshell-prompt-regexp

This new default makes eshell-prompt-regexp's consumers a little less
susceptible to false positives in garbage/process output and a little
more resistant to user changes to eshell-prompt-function. It's also
closer to its default value (KISS).

modules/term/eshell/config.el

@@ -69,7 +69,7 @@ You should use `set-eshell-alias!' to change this.")
         ;; TODO Use `eshell-input-filter-initial-space' when Emacs 25 support is dropped
         eshell-input-filter (lambda (input) (not (string-match-p "\\`\\s-+" input)))
         ;; em-prompt
-        eshell-prompt-regexp "^.* λ "
+        eshell-prompt-regexp "^[^#$\n]* [#$λ] "
         eshell-prompt-function #'+eshell-default-prompt-fn
         ;; em-glob
         eshell-glob-case-insensitive t