add Object:is() to luals definition

def/Object.d.lua

@@ -21,4 +21,9 @@ function Object:extend() end
 ---@param mixin table
 function Object:implement(mixin) end
 
+---Checks if object is of a specific type
+---@param object Object
+---@return boolean
+function Object:is(object) end
+
 return Object