string.unpack vararg returns

還沒有人認領這個 Issue。

### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Linux

### What is the issue affecting?

Annotations, Type Checking, Completion

### Expected Behaviour

The current `string.unpack` annotations (for the returns) are as following:

```lua
---@param fmt string
---@param s string
---@param pos? integer
---@return any ...
---@return integer offset
---@nodiscard
function string.unpack(fmt, s, pos) end
```

The function will indeed return any number of `any` returns followed by a single integer return for the new offset. As such you would expect that in the following example code, all of the returns are `any` except the last return be `integer`:

```lua
local a, b, c, d, offset = string.unpack('I2I2I2I2', "some binary data")
```

### Actual Behaviour

Instead of the `a, b, c, d` being typed as `any` and `offset` being typed as `integer`, we instead see that `a` is typed as `any`, `b` is typed as `integer`, and `c, d, offset` are typed as `nil`.

This happen because it is not supported for a function to return a vararg followed by any return; it is always that vararg returns are the last return. Because of that, only the first return is typed as `any` and from there the second `@return` takes.

### Reproduction steps

1. Copy the example code.
2. Hover over each return variable to notice the type.

### Additional Notes

_No response_

### Log File

_No response_

從 string.unpack 註解開始，並在 Visual Studio Code 擴充功能中將滑鼠停留在每個回傳的變數上，以重現此問題。追蹤型別檢查器如何處理後接 offset 回傳值的 vararg。完成的標準是 unpack 值的型別為 any，且最後的 offset 型別為 integer。

由索引模型根據 Issue 內容生成。