Expose js2-parsed-errors and js2-parsed-warnings in the mode-line

[lelit]

In Emacs 26, both comint and the new flymake use a nice display of this kind of information in the mode-line. I'm currently using the following customization:

(add-hook 'js2-parse-finished-hook
          (lambda ()
            (setq mode-name "JS2")
            (when (> (length js2-parsed-errors) 0)
              (setq mode-name
                    (list mode-name
                          ":"
                          (propertize (format "%dE" (length js2-parsed-errors))
                                      'face 'error))))
            (when (> (length js2-parsed-warnings) 0)
              (setq mode-name
                    (list mode-name
                          ":"
                          (propertize (format "%dW" (length js2-parsed-warnings))
                                      'face 'warning))))
            (force-mode-line-update)))

but it would be more consistent and nicer using the standard properties to decorate the details, possibly attaching a menu offering next-error and previous-error (flymake binds those to mouse-4 and mouse-5...).