fix: typed @field should not override other defined field

[tomlau10]

fixes #2171, fixes #2711

The problem

I found these issues while discussing in #3221.
In short, the typed @field [T1] T2 should only apply when no types found for that specific key.
I found a fix after some debugging, implementation details: #3221 (comment)

before

---@class A
---@field [string] boolean
local A
function A:f() end

local a = A.a   --> boolean ✅
local f = A.f   --> boolean|function ❌

after

---@class A
---@field [string] boolean
local A
function A:f() end

local a = A.a   --> boolean ✅
local f = A.f   --> function ✅

中文版

@field [T1] T2 似應該只在對應 key 找不到任何 type define 時才 apply
解決方式是簡單將相關 infer logic 延後到查找完 class variable 的 setfield/setmethod 之後才處理