feature: Variadic generic packs and mapping expansion

Nobody has claimed this yet.

#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:
```lua
---@generic R, T...
---@param fn fun(...: T...):R
---@param ... Component
---@return System
function System.new(fn, ...)
...
end

---@generic R
---@param System 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.

Written by the indexing model from the issue text.