Fix flymake mode line scrolling with pixel-scroll-precision-mode

Fix flymake mode line scrolling with pixel-scroll-precision-mode

When pixel-scroll-precision-mode is enabled, scrolling the mouse
wheel will yield wheel-{up,down} events.  Flymake now binds the
new events in addition to the old mouse-wheel-{up,down}-event.

* lisp/progmodes/flymake.el:(flymake--mode-line-counter-scroll-prev)
(flymake--mode-line-counter-scroll-next)
flymake--mode-line-counter-map): New.
(flymake--mode-line-counter): Use new keymap and include
'flymake--diagnostic-type' as a property in the mode-line.
(Bug#64428)