Support for PHP->JS->PHP data transfer by returning value from post_message_to_js()

[adamziel]

Description

A part of #724

post_message_to_js passes a message to the JavaScript module where it can be handled inside onMessage(). This commit adds support for returning values and promises inside that handler to feed them back to PHP. For example:

$response = post_message_to_js(json_encode($request_data));

	await playground.onMessage(async (message: string) => {
		const requestData = JSON.parse(message);
		const response = await fetch(requestData);
		return JSON.stringify(toSimpleObject(response));
	});

A handler like above could be used to add support for network requests, which is in fact what #724 does

[adamziel]

I'll go ahead and merge this feature alone without merging support for network requests.