fix: missing field completion for generic class object

[tomlau10]

fixes #2196, fixes #2945
resolves #3041 (fixing this bug fulfills the requirement of that issue, such that the feature request becomes unnecessary)

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Bug explanation: #2945 (comment)

Example after bugfix

---@class Array<T>: { [integer]: T }
---@field private length integer
local Array

function Array:push()
    self.  --> suggest `length`, `push(self)`
end

---@type Array<string>
local a

print(a.length) --> Field `length` is private, it can only be accessed in class `Array`.

a: --> suggest `push()`

Product placement

CppCXY's new lua language server written in rust has better generic support 😃

• [PREVIEW] a new lua language server #3017

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中文版

簡單在 searchFieldSwitch 增加對 doc.type.sign 的支持，以修復 generic class object 沒有 field completion 的問題。
目前其實還有一個問題，就是 field completion 還會列出 integer 😕
似是由 { [integer]: T } 獲取出來的 integer key field name

---@class Array<T>: { [integer]: T }

---@type Array<string>
local a
a. --> why suggest `integer` ??

但由於這個問題本身已存在，且我暫時也不清楚原因，就先不在此 PR 糾結了