Add Flutter web renderer information to event

[ueman]

One can detect wether Flutter for Web runs with the HTML renderer or the CanvasKit renderer like this:

import 'dart:js' as js;

final dynamic flutterCanvasKit = js.context['flutterCanvasKit'];
if(flutterCanvasKit != null){
// runs with CanvasKit renderer
}

This is especially helpful for bugs where the UI isn't renderer right.

Docs on the web renderer: https://docs.flutter.dev/development/tools/web-renderers

Since this issue is already quite old, the proposed solution above needs to be double-checked to see if it's still valid.