feature suggestion: add a notation for callbacks that expects the user to implement

There exist some callbacks in the LÖVE framework, like love.conf, love.draw and some others, and there is currently no way in EmmyLua to note that they are callbacks rather than normal functions, and that a new definition of the same function is an implementation rather than a new overload.

Current Behavior

No way to indicate that this is a callback instead of a normal function definition.

-- In the API definition.
---xxx
---@param t string
function callback(t) end


-- In user code
function callback(t)
    t:$
end

There is no completion based on the type of the parameter or ...

Suggested Behavior

Add a new notation for callbacks so when user is defining a new implementation, they can get completion on the parameters. This is especially useful when the parameter is extremely complex (for an example, see love.conf)

-- In the API definition.
---@callback
---xxx
---@param t string
callback = nil

-- In the user code
function callback(t)
    t:$
end

I am totally open to the specific syntax, and I think what is really important is to have a syntax for them.

首先追踪 EmmyLua annotations 和函数定义的表示方式，然后将其与 issue 中提到的 LÖVE 回调进行比较。确定回调语法，以及它与用户实现和参数补全之间的交互方式。当该记法能够区分回调和普通函数，并支持回调参数补全时，即视为完成；不指定任何文件或测试。