Incorrect field type calculation when field name in variable

还没有人认领这个 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

```

首先使用代码示例重现 Lua language server 中的类型检查差异，比较字面量索引与通过 `field` 变量进行的索引。跟踪字段类型推断的入口点，并为所示案例添加回归测试；当基于变量的索引不再报告 `Class`，而字面量索引报告 `unknown` 时，即表示完成。

由索引模型根据 Issue 内容生成。