Improve squiggly heredoc support in non-SMIE Ruby mode

* lisp/progmodes/ruby-mode.el (ruby-parse-partial): Support
squiggly heredocs here, too (port from upstream).

lisp/progmodes/ruby-mode.el

@@ -1151,7 +1151,7 @@ delimiter."
        ((looking-at "<<")
         (cond
          ((and (ruby-expr-beg 'heredoc)
-               (looking-at "<<\\(-\\)?\\(\\([\"'`]\\)\\([^\n]+?\\)\\3\\|\\(?:\\sw\\|\\s_\\)+\\)"))
+               (looking-at "<<\\([-~]\\)?\\(\\([\"'`]\\)\\([^\n]+?\\)\\3\\|\\(?:\\sw\\|\\s_\\)+\\)"))
           (setq re (regexp-quote (or (match-string 4) (match-string 2))))
           (if (match-beginning 1) (setq re (concat "\\s *" re)))
           (let* ((id-end (goto-char (match-end 0)))