Add a way of marking a function as overidable

Some engines/libraries let user override global functions, but then adding API headers for them gives a "duplicate field" warning.

eg

Header file for engine API:

--- Engine API
global_table = {}

---Overidable function
---@param test_data string
---@return boolean
function global_table.do_stuff(test_data) end

Code that use this engine:

--- Currently raises the duplicated field warning
function global_table.do_stuff(test_data)
    -- Handle stuff differently
end 

该 issue 未指定任何文件、测试或入口点。首先追踪 Lua 函数的重复字段警告是如何产生的，并检查现有的注解或覆盖处理。完成标准是：有一种记录在文档中的方式，可将 API 函数标记为可覆盖，这样用户实现就不再触发重复字段警告。