Child iframes (iframes within the activity iframe) shouldn't be allowed to communicate with DiscordSDK.

[real2two]

I'm currently working on an activity involving iframes...

When I was trying to send a message to the activity iframe (<Window>.postMessage) from a (same-origin) child iframe, DiscordSDK would pick it up on the handleMessage event as well, so I ran into these errors:

I don't want the child iframe from being able to send messages to DiscordSDK, so it would be nice if there was an option to add if (event.source !== this.source) return; to disallow this from happening below this code segment:

embedded-app-sdk/src/Discord.ts

Lines 275 to 276 in 2cb879a

	private handleMessage = (event: MessageEvent) => {
	if (!ALLOWED_ORIGINS.has(event.origin)) return;

[real2two]

(whoops. ignore that. I accidently closed it while forking the repository.)

[real2two]

I created a PR to fix this. #284