[webview_flutter_web] Improve support for JavaScript channels

[Frank3K]

An improvement on #1.

Note: base branch is feat/javascript-channels.

Improvements

Security

Incoming messages from other origins are now ignored, as advised on MDN.

Messages processing in case of multiple webview

Previously, if one created N webviews, each message was being processed N times. This can now be prevented by choosing a unique channel name per webview. From web, messages should be send in a slightly different way.

Before:

	window.parent.postMessage(message, targetOrigin);

New format:

	window.parent.postMessage(
		{
			channel: this.channelName,
			message: message,
		},
		targetOrigin
	);

[timovandeput]

LGTM (see one suggestion)

[timovandeput]

No need for the explicit variable type, as the compiler can infer it from "as String?".

[Frank3K]

This repo has stricter analysis options; I received a number of warnings otherwise.