Warn on adding listeners after bot start

[MKRhere]

New users of Telegraf often mistakenly write listeners within listeners like so:

bot.command("form", async ctx => {
	await ctx.reply("Send first input");
	bot.on("message", async ctx => {
//	^^^ BUG! This attaches a new global listener every time!
		await ctx.reply("Send second input");
		...
	});
});

When exploring ideas to resolve this, @trgwii suggested that we log a warning when we detect new listeners added after bot.launch() (perhaps after the first bot.handleUpdate() to account for webhooks?). We don't really need to track all composers, because this problem almost exclusively happens with the base Telegraf instance. The warning can link to a doc page about this specific behaviour with alternatives (Wizard, ...?) and ways to disable the warning if they know what they're doing.

Before implementing this, I'd like to know if any plugins rely on this kind of behaviour at runtime. We may not want to go through with this if any popular plugin attaches listeners on Telegraf at runtime for a valid reason (Is there one? Ready to discuss).