[web] Turn on CanvasKit Chromium

[mdebbar]

This is the last bit to make CanvasKit Chromium available to our users by default.

Users can always opt-out by canvasKitVariant: "full" to their runtime configuration similar to this.

_flutter.loader.loadEntrypoint({
  onEntrypointLoaded: async function(engineInitializer) {
    let appRunner = await engineInitializer.initializeEngine({
      canvasKitVariant: "full",
    });

    await appRunner.runApp();
  }
});

Closes flutter/flutter#122329

[kevmoo]

Do we have full and complete documentation of all of the initializeEngine options...somewhere?

[yjbanov]

[mdebbar]

Do we have full and complete documentation of all of the initializeEngine options...somewhere?

Yes, here: https://docs.flutter.dev/development/platform-integration/web/initialization#initializing-the-engine

Filed an issue to add documentation for canvasKitVariant to make sure I don't forget: flutter/flutter#123048