feat: add 'mousescroll' option (#12355)

Add 'mousescroll' option to control how many lines to scroll by when a
mouse wheel keycode is received. The mousescroll option controls both
horizontal and vertical scrolling. The option is a string in the format:

    set mousescroll=direction:count,direction:count

Where direction is either "ver" or "hor", and count is a non negative
integer. If a direction is omitted, a default value is used. The default
values remain unchanged, that is 3 for vertical scrolling, and 6 for
horizontal scrolling. As such, the mousescroll default is "ver:3,hor:6".

Add mousescroll documentation
 - Add option documentation in options.txt
 - Add brief summary in quickref.txt

Update :help scroll-mouse-wheel
 - Mention mousescroll option as a means of controlling scrolling.
 - Remove obsolete suggestion to map scroll wheel keys to <C-U> to
   scroll by a single line -- users should prefer the mousescroll option.
 - Add some information about the consequences of remapping scroll wheel
   keys (they lose their magic ability to affect inactive windows).

Update :help vim-differences
 - Add brief mousescroll summary under Options

Add mousescroll tests
 - Test option validation
 - Test default mousescroll value and behavior
 - Test fallback to default values
 - Test mouse vertical and horizontal scrolling in normal mode
 - Test mouse vertical and horizontal scrolling in insert mode

src/nvim/normal.c

@@ -3392,8 +3392,8 @@ static void nv_mousescroll(cmdarg_T *cap)
     if (mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL)) {
       (void)onepage(cap->arg ? FORWARD : BACKWARD, 1L);
     } else {
-      cap->count1 = 3;
-      cap->count0 = 3;
+      cap->count1 = p_mousescroll_vert;
+      cap->count0 = p_mousescroll_vert;
       nv_scroll_line(cap);
     }
   } else {