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_

Start with the string.unpack annotation and reproduce the issue in the Visual Studio Code extension by hovering over each returned variable. Trace how the type checker handles a vararg followed by the offset return. Done means the unpack values are typed as any and the final offset is typed as integer.

Written by the indexing model from the issue text.