Make some Update sub-types generic

[wojpawlik]

To allow types like Context<MessageUpdate<TextMessage>> in the future; Telegraf can't use this right now due to microsoft/TypeScript#4742, so don't bother releasing just this on npm.

[KnorpelSenf]

This is rather on the side of useful helper types, it does not directly reflect the Telegram API. I am not sure if this is in scope of typegram or if it should rather be implemented by dependent libraries, e.g. like this:

type Upd<M extends Message> = Update.MessageUpdate & { message: M }

I will keep this open until we have a concrete use-case for it, maybe then it is clearer whether this is the right place to implement your idea.

[wojpawlik]

Use case: ctx in bot.on('text', ctx => { ... }) should be Context<MessageUpdate<TextMessage>>