File Viewer Flutter macOS adapter

Internal WKWebView adapter for file_viewer_flutter. Use the app-facing package instead of depending on this implementation directly.

The retained WKWebView moves between fresh native containers when Flutter routes reattach. A deferred disposal of an old platform-view ID can therefore only remove its old container. The rendering engine and document stay alive.

Derived from the immutable public flutter_inappwebview_macos 1.2.0-beta.3 release under Apache-2.0. See LICENSE and UPSTREAM.json. Reproduce the sources with python3 tool/vendor-macos.py from the repository root. The package, native plugin class and platform channel namespaces are isolated from the upstream plugin so applications can also use another InAppWebView instance.