whitespace.el: New whitespace style `lines-char'

whitespace.el: New whitespace style `lines-char'

* lisp/whitespace.el (whitespace-style):
(whitespace-style-value-list):
(whitespace-toggle-option-alist):
(global-whitespace-toggle-options):
(whitespace-help-text):
(whitespace-interactive-char):
(whitespace-style-face-p):
(whitespace-color-on):
(whitespace-lines-regexp): New whitespace style `lines-char', which
highlights only the first character over the whitespace-line-column
limit (bug#55980).