Disallow ? on values

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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?

Type Checking

### Expected Behaviour

`?` is not a type, and should only be used when marking fields as optional. If nil is explicitly allowed as a value, `| nil` should be used. This is in line with TypeScript.

### Actual Behaviour

`?` does not add `| nil` to the value type, and the second option should be disallowed.

### Reproduction steps

```lua
---@class Foo
---@field foo? string
---@field bar string?
---@field baz string | nil
local test = {};

function Foo()
return test.foo, test.bar, test.baz
end;
```

### Additional Notes

_No response_

### Log File

_No response_

Beginne damit, das Verhalten der Typprüfung in der Visual Studio Code Extension (sumneko.lua) anhand des Lua-Snippets im Issue zu reproduzieren. Fertig ist es, wenn `string?` nicht zulässig ist, `foo? string` weiterhin die Form für optionale Felder bleibt und `string | nil` nil ausdrücklich zulässt.

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