Proposal consider vararg returns as forced optional

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

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Windows

### What is the issue affecting?

Diagnostics/Syntax Checking

### Expected Behaviour

I suggest consider **vararg returns** as **optional** (can be `nil`) because no one guarantees that they will be filled in. The same goes for arguments, despite the fact that their type can also be optional or nilable.

If we return at least something, but we don't know the quantity, then the first argument won't be vararg - `any, any ...`.

Thus after that we do not need to additionally specify that the vararg returns is optional.

```lua
---@param a string
function test(a) end

---@param ... string
---@return string ...
function f(...) end
local a, b, c = f() --no warn
local a, b, c = f("1")

test(a) --no warn, expected optinal cannot match

---@param ... string?
---@return string? ...
function f(...) end
local a, b, c = f() --no warn
local a, b, c = f("1")

test(a) --warn optinal cannot match

---@param ... string|nil
---@return string|nil ...
function f(...) end
local a, b, c = f() --no warn
local a, b, c = f("1")

test(a) --warn nil cannot match
```

### Actual Behaviour

Vararg arguments are optional, but vararg returns are not.


### Reproduction steps

1. Make vararg return of some existing type.
2. It is guaranteed to be infinite - not optional or nil.

### Additional Notes

_No response_

### Log File

_No response_

Start with the Lua language server's diagnostics and type-inference handling for vararg arguments and returns, using the reproduction steps and examples as the behavioral guide. Verify that vararg returns are treated as optional or nilable as appropriate, while the provided calls produce the expected warnings and no warnings.

Written by the indexing model from the issue text.