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.

Start with the related issue #1861 and the proposed Lua examples in this issue. Define how variadic generic packs, mapping expansion, and the ... shorthand should behave, then verify that the demonstrated System API can express typed component parameters and return values.