[Feature] HomeWidget.isSupported()

[nielsbasjes]

Because having a HomeWidget is only supported on certain platforms I propose a new function bool HomeWidget.isSupported() which simply returns if the current runtime of the application supports having a HomeWidget.

This wil make it easier for applications to know if the code fragments that build this widget is to be initialized/called.

I'm not 100% sure but I suspect the code can be as simple as this (perhaps even cache the value)

bool isSupported() {
  if (kIsWeb) {
    return false;
  }
  return (defaultTargetPlatform == TargetPlatform.iOS || defaultTargetPlatform == TargetPlatform.android);
}