input.conf: bind Ctrl+WHEEL_UP/DOWN to video-zoom

There are good reasons to bind Ctrl+WHEEL_UP/WHEEL_DOWN to video-zoom:

- They are ubiquitous and familiar key bindings to represent zooming
operations, which are used in all popular web browsers, document viewers,
and document editors.
- Because WHEEL_UP/WHEEL_DOWN are scaled with high-resolution scrolling
input devices like touchpads, this allows smooth zooming.
- This makes "pinch to zoom" with touchpads and touchscreens work out of
box on Windows, since by default applications receive these key inputs
for pinch gesture.
- It had been considered to bind these keys to window-scale instead.
However, this results in horrible UX as the keybinds work only when the
mouse pointer is over the mpv window, and if the window shrinks during
this operation, the window below mpv now receives these keybinds,
resulting in unwanted zooming for that window, which violates the principle
of least surprise.