Better support for varargs in functions in classes declared with @field

I ran into a minor problem with how vararg is handled in function declarations in an field annotation when the function declaration itself later explicitly lists the arguments.

---@class Handler
---@field ReceiveMsg fun(self: Handler, type: number, ...: string)
local handler = {}

function handler:ReceiveMsg(arg1, arg2, arg3, arg4)
 --- arg2 is inferred as string, but arg3 and arg4 are "any"
end

Why do I even have this mismatch between field and function declaration? The field annotation comes from a library and is an event handler -- the number of parameters depend on a lot of things, but for a given implementation you usually only care about a few that you explicitly declare.

まず、@field ReceiveMsg vararg アノテーションと明示的なメソッド宣言を使って、issue の Handler 例を再現します。関数宣言とパラメータ推論の間で vararg 型がどのように伝播するかを追跡します。完了条件は、arg2、arg3、arg4 が一貫して string として推論され、後続の引数が any のままにならないことです。