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.

Comienza con el ejemplo de ActionType del issue y lee la documentación enlazada de TypeScript sobre el estrechamiento de tipos para definir el comportamiento esperado. Se considera terminado cuando los valores de una unión exponen solo las propiedades compartidas antes de una comprobación de tipos, mientras que las comprobaciones del campo type exponen las propiedades de la variante de acción correspondiente.