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.

まず、EmmyLua annotations と関数定義がどのように表現されているかを追跡し、それを issue で名前が挙げられている LÖVE コールバックと比較します。コールバック構文と、それがユーザー実装およびパラメーター補完とどのように相互作用するかを決定します。記法によってコールバックと通常の関数が区別され、コールバックパラメーターの補完が可能になれば完了です。ファイルやテストの名前は挙げません。