[Feature request] Annotation for referring to superclass

還沒有人認領這個 Issue。

I'm trying to add annotations to [classic](https://github.com/rxi/classic/blob/master/classic.lua), but encounter a few problems when doing so.

For example, the `super` field refers to the superclass. If I were to annotate super like this
```lua
---@class Object
---@field protected super Object
```
then subclasses will have a `super` field, but it will always refer to `Object`, instead of their superclass. The best solution I can come up with the current implementation is to set the `super` field each time a class is created.
```lua
---@class Animal : Object
---@field super Object
local Animal = Object:extend()

---@class Bird : Animal
---@field super Animal
local Bird = Object:extend()
```
This is a repetition of steps, which tells me DRY. I suggest something like a `super` annotation, which always refers to the superclass relative to which class it is being used.
```lua
---@class Object
---@field protected super super

---@class Animal : Object
local Animal = Object:extend()
-- Animal.super refers to Object

---@class Bird: Object
local Bird = Animal:extend()
-- Bird.super refers to Animal
```

先檢視 issue 中參照的 annotation 範例與 classic.lua 繼承模型。確認類別欄位 annotation 在何處解析，並定義相對於超類別的 annotation 對 Object、Animal 和 Bird 應有的行為；issue 未指定任何 LuaLS 檔案或測試，因此必須在實作前建立完成條件。

由索引模型根據 Issue 內容生成。