positioning.lua: add this script

This script provides script bindings to pan videos and images, making
mpv a better image viewer out of the box.

Fixes #3038, fixes #15020.

player/lua.c

@@ -84,6 +84,9 @@ static const char * const builtin_lua_scripts[][2] = {
     },
     {"@select.lua",
 #   include "player/lua/select.lua.inc"
+    },
+    {"@positioning.lua",
+#   include "player/lua/positioning.lua.inc"
     },
     {0}
 };