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.

Start with the issue's ActionType example and read the linked TypeScript narrowing documentation to define the expected behavior. Done means union values expose only shared properties before a type check, while checks on the type field expose the properties of the matching action variant.