Disallow ? on values

还没有人认领这个 Issue。

### 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_

首先，使用 issue 中的 Lua 代码片段复现 Visual Studio Code Extension (sumneko.lua) 中的类型检查行为。完成的标准是：不允许使用 `string?`，`foo? string` 仍然表示可选字段形式，并且 `string | nil` 明确允许 nil。

由索引模型根据 Issue 内容生成。