string.unpack vararg returns

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### 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_

Commencez par l’annotation de string.unpack et reproduisez le problème dans l’extension Visual Studio Code en survolant chaque variable retournée. Suivez la manière dont le vérificateur de types traite un vararg suivi du retour de l’offset. Le travail est terminé lorsque les valeurs de unpack sont typées en any et que l’offset final est typé en integer.

Rédigé par le modèle d'indexation à partir du texte de l'issue.