[desktop_webview_window] Add support for registerJavaScriptMessageHandler on Linux

[Carapacik]

registerJavaScriptMessageHandler and unregisterJavaScriptMessageHandler on Linux

Example of usage

<button onclick="window.webkit.messageHandlers.onTokenReceived.postMessage('test-token')">
            Send Token
</button>

Reopen #428

[Copilot]

Pull request overview

This PR adds support for registerJavaScriptMessageHandler and unregisterJavaScriptMessageHandler on Linux, allowing JavaScript in webviews to communicate with Flutter code using the WebKit message handler API (window.webkit.messageHandlers.<handlerName>.postMessage()).

Key changes:

• Implemented Linux platform support for JavaScript message handlers in C++ using WebKit's user content manager API
• Updated Dart code to enable these methods on Linux in addition to macOS
• Bumped package version from 0.2.4 to 0.2.5

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
packages/desktop_webview_window/lib/src/webview_impl.dart	Modified platform checks to enable JavaScript message handlers on Linux; attempted to clean up null safety code
packages/desktop_webview_window/linux/webview_window.h	Added method declarations for RegisterJavaScriptChannel and UnregisterJavaScriptChannel
packages/desktop_webview_window/linux/webview_window.cc	Implemented JavaScript message handler registration/unregistration using WebKit APIs
packages/desktop_webview_window/linux/desktop_webview_window_plugin.cc	Added method call handlers for registerJavaScripInterface and unregisterJavaScripInterface
packages/desktop_webview_window/pubspec.yaml	Bumped version to 0.2.5 and relaxed flutter_lints constraint
packages/desktop_webview_window/CHANGELOG.md	Documented the new Linux support for JavaScript message handlers
packages/desktop_webview_window/example/pubspec.lock	Updated dependency versions
packages/desktop_webview_window/example/macos/Flutter/GeneratedPluginRegistrant.swift	Updated plugin import from path_provider_macos to path_provider_foundation

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[Copilot]

When unregistering a JavaScript channel, the signal handler connected in RegisterJavaScriptChannel is not disconnected, leading to a memory leak. The HandlerData allocated on line 383 will never be freed after unregistration because the destroy notify callback on line 419-421 will not be invoked.

Consider storing the signal handler ID returned by g_signal_connect_data and calling g_signal_handler_disconnect in UnregisterJavaScriptChannel before unregistering the script message handler.

[boyan01]

LGTM, Thanks for the PR

[Carapacik]

@boyan01 When will you publish the version on pub.dev?