Macos builds fail to show webview due to unsupported call

[8bitcrag]

I believe this is the cause of #9 and possibly #3, depending on platform.

The following part of the webview_mobile_and_mac code calls a function that is not supported on macos:

flet-webview/src/flutter/flet_webview/lib/src/webview_mobile_and_mac.dart

Line 74 in b1cfb31

controller.setOnScrollPositionChange((ScrollPositionChange position) {

In the flutter project this is shown here:

https://github.com/flutter/packages/blob/0ec405358e43fd3cca4f267264b5d7fa4a755498/packages/webview_flutter/webview_flutter_wkwebview/lib/src/webkit_webview_controller.dart#L785

I would submit a PR, but I'm unsure what preferred approach would be. For example: a platform check within webview_mobile_and_mac, a new webview version just for macos, a passed in flag, etc