Basic partial class support

[tomlau10]

resolves #3023, resolves #2561

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Since #2970, the partial class "hack" for not checking missing inherited fields no longer works.
This PR brings back this feature by introducing a partial attribute for @class.
A class marked with partial will not check missing inherited fields, just like the original checking behavior.

Example

---@class Config
---@field a number

---@class (partial) Config.P: Config
---@field b number

---@type Config.P[]
local cfgs = {}
cfgs[1] = { b = 1 } -- no warning
cfgs[2] = {}        -- only warns missing `b`

中文版

自 #2970 起，原有的 partial class hack 被封了。
這個 PR 對 @class 簡單地引入 partial 屬性
=> 帶 partial 屬性的 class 在檢查 missing fields 時會忽略 inherited class 的字段，就像之前的 check logic 一樣

[sumneko]

Thank you!