literal strings 'async' is mis-parsed

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?

Linux

### What is the issue affecting?

Annotations

### Expected Behaviour

The following should work without errors:
```lua
---@class Unit
---@field mode "async" | "sync"
```

and:
```lua
---@type Unit
local test = {}

test.mode = (hint) --> "async" or "sync"
```

### Actual Behaviour

The follow shows an error
```lua
---@class Unit
---@field mode "async" (error here: expected.) | "sync"
```

and:
```lua
---@type Unit
local test = {}

test.mode = (hint) --> "sync"
```

### Reproduction steps

1. Copy and paste the expected behavior codeblock.

### Additional Notes

Only happens with `async` as far as I can see.

### Log File

_No response_

Start by reproducing the annotation examples in the Visual Studio Code extension and compare how the parser handles the literal string "async" with "sync". Trace the annotation/type-name handling that produces the expected error, then add coverage showing both literal values are accepted without changing the reported type hint.

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