feature: Variadic generic packs and mapping expansion

#1861

This feature request requests variadic generics functionality, as well as mapping those generics. My proposed implementation of this feature request would allow the following example syntax:

---@generic R, T...
---@param fn fun(...: T...):R
---@param ... Component<T>
---@return System<R>
function System.new(fn, ...)
    ...
end

---@generic R
---@param System<R> self
---@param entities Entity[]
---@return R[]
function System:run()
    ...
end

The example code above also introduces a shorthand for variadics (line 3): whenever the parameter ... would have an unexpanded parameter pack as an argument, it is implicitly expanded. Any type can be expanded at any point by suffixing a ..., as shown in line 2.

The motivation of this request arises from an well-typed ECS implementation with object-oriented systems. Without variadic generics, it is not possible in any way I know of to allow a system to depend on any amount of components and still have well-typed parameters.

関連する issue #1861 と、この issue で提案されている Lua の例から始めます。可変長ジェネリックパック、マッピング展開、... の省略記法がどのように振る舞うべきかを定義し、次に、示されている System API で型付きのコンポーネントパラメーターと戻り値を表現できることを検証します。