Incorrect field type calculation when field name in variable

還沒有人認領這個 Issue。

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

Visual Studio Code Extension (sumneko.lua) v3.11.0

### Which OS are you using?

Linux

### What is the issue affecting?

Type Checking

### Reproduction steps

```lua
---@diagnostic disable: unused-local

---@class Class
local Class = { __type = "Class" }

local field = "some_string" -- field: string = "some_string"

do
local c = {} ---@type Class

-- OK
local f1 = c["some_string"] -- f1: unknown
local f2 = c[field] -- f2: unknown
end

function Class:f()
-- OK
local f1 = self["some_string"] -- f1: unknown

-- BUT
local f2 = self[field] -- f2: string = "Class"
end

do
-- OK
local f1 = Class["some_string"] -- f1: unknown

-- BUT
local f2 = Class[field] -- f2: string = "Class"
end

```

先使用程式碼範例重現 Lua language server 中的型別檢查差異，比較字面值索引與透過 `field` 變數進行的索引。追蹤欄位型別推論的進入點，並為所示案例新增回歸測試；當以變數為基礎的索引不再回報 `Class`，而字面值索引回報 `unknown` 時，即表示完成。

由索引模型根據 Issue 內容生成。