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/option_defs.h

@@ -153,6 +153,11 @@
 #define MOUSE_NONE      ' '             // don't use Visual selection
 #define MOUSE_NONEF     'x'             // forced modeless selection
 
+// default vertical and horizontal mouse scroll values.
+// Note: This should be in sync with the default mousescroll option.
+#define MOUSESCROLL_VERT_DFLT   3
+#define MOUSESCROLL_HOR_DFLT    6
+
 #define COCU_ALL        "nvic"          // flags for 'concealcursor'
 
 /// characters for p_shm option:
@@ -528,6 +533,9 @@ EXTERN long p_mls;              // 'modelines'
 EXTERN char_u *p_mouse;       // 'mouse'
 EXTERN char_u *p_mousem;      // 'mousemodel'
 EXTERN int p_mousef;           // 'mousefocus'
+EXTERN char_u *p_mousescroll;  // 'mousescroll'
+EXTERN long p_mousescroll_vert INIT(= MOUSESCROLL_VERT_DFLT);
+EXTERN long p_mousescroll_hor INIT(= MOUSESCROLL_HOR_DFLT);
 EXTERN long p_mouset;           // 'mousetime'
 EXTERN int p_more;              // 'more'
 EXTERN char_u *p_opfunc;      // 'operatorfunc'