support windows platforms

[DanielAvdar]

This pull request adds official Windows support to the flet_webview package and introduces a new implementation for the WebView on Windows and Linux platforms. The main focus is on enabling and improving Windows compatibility, including platform-specific initialization, event handling, and method invocation for the desktop WebView component.

Platform support updates:

• Windows is now officially marked as supported in the README.md platform matrix.

Desktop WebView implementation improvements:

• The WebviewDesktop widget is converted from a stateless to a stateful widget, now requiring a control parameter for event and method handling. [1] [2]
• On Windows, the widget initializes a webview_windows.WebviewController, sets up event listeners for URL changes and loading state, and supports various control methods (reload, navigation, cache/cookie clearing, JavaScript execution, etc.).
• On Linux, the widget displays an informative error message about lack of support, including a link to further information.
• The widget now shows a loading indicator while initializing, and displays error messages if initialization fails.

Dependency updates:

• Added the webview_windows package to pubspec.yaml to support the new Windows WebView implementation.

Integration and usage:

• The instantiation of WebviewDesktop in webview.dart is updated to pass the required control parameter, ensuring proper integration with the new stateful widget.

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