[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

该 issue 没有指定文件、测试或入口点。首先将请求的 @guard 注解和 isString 示例与语言服务器现有的类型收窄行为进行比较。用户定义的 guard 能在 true 分支将 x 收窄为 string，并在 false 分支将其收窄为 number，即表示完成。