[Feature request] User defined type guards like TypeScript's one

As a simple example, I would like to annotate the following @guard x string. This is in effect returning a boolean, which narrows down the type of the argument x.

---@param x unknown
---@guard x string
local function isString(x)
  return type(x) == "string"
end

---@param x string | number
local function main(x)
  if isString(x) then
    -- x is string
    print(x)
  else
    -- x is number
    print(x)
  end
end

該 issue 沒有指定檔案、測試或進入點。首先將要求的 @guard 註解和 isString 範例與語言伺服器現有的型別縮窄行為進行比較。使用者定義的 guard 能在 true 分支將 x 縮窄為 string，並在 false 分支將其縮窄為 number，即表示完成。