vim-patch:8.2.4674: cannot force getting MouseMove events

Problem:    Cannot force getting MouseMove events.
Solution:   Add the 'mousemoveevent' option with implementaiton for the GUI.
            (Ernie Rael, closes vim/vim#10044)
c4cb544cd5

This only ports the docs and the option variable.
The following commits will actually implement it as a UI option.

src/nvim/option_defs.h

@@ -616,6 +616,7 @@ EXTERN int p_ma;                ///< 'modifiable'
 EXTERN int p_mod;               ///< 'modified'
 EXTERN char *p_mouse;           // 'mouse'
 EXTERN char *p_mousem;          // 'mousemodel'
+EXTERN int p_mousemev;          ///< 'mousemoveevent'
 EXTERN int p_mousef;            // 'mousefocus'
 EXTERN char *p_mousescroll;     // 'mousescroll'
 EXTERN long p_mousescroll_vert INIT(= MOUSESCROLL_VERT_DFLT);