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_

Start by reproducing the type-checking behavior in the Visual Studio Code Extension (sumneko.lua) using the Lua snippet in the issue. Done means `string?` is disallowed, `foo? string` remains the optional-field form, and `string | nil` explicitly permits nil.

Written by the indexing model from the issue text.