Support discriminated unions / type narrowing

In TypeScript this is supported by "narrowing"

Consider this example:

---@alias ChatAction { type: '"chat"', targetName: string, msg: string }
---@alias NotifyAction { type: '"notify"', msg: string, colour: number }
---@alias ActionType ChatAction | NotifyAction

---@param action ActionType
local function example(action)
    -- action is any action (properties 'type', 'msg')
    if (action.type == 'chat') then
        -- action is ChatAction (properties 'type', 'msg', 'targetName')
    end
    if (action.type == 'notify') then
        -- action is NotifyAction (properties 'type', 'msg', 'colour')
    end
end

For a union type such as 'ActionType' in the example, the only defined properties should be those common to all types. After narrowing (through some sort of type check, in this case a type field) the additional properties in the narrowed subset are also available.

从 issue 的 ActionType 示例开始，并阅读链接的 TypeScript 类型收窄文档，以定义预期行为。完成的标准是：在进行类型检查之前，联合值只暴露共享属性；而对 type 字段进行检查时，则暴露匹配的 action variant 的属性。