feat: add custom CEF V8 extensions support (#17)

* feat: Register extensions in render process

- Add CefExtensions type to hold V8 extension code
- Pass extensions through BrowserProcessAppBuilder
- Register extensions in RenderProcessHandler on WebKit initialization
- Decode JSON extensions from command line switch
- Prefix extension names with "v8/" per CEF convention
- Include actual JSON in error messages for debugging

l

* feat: refactor window.cef API and register as CEF extension

* fix: remove debug print statements in render process handler

* refactor: centralize EXTENSIONS_SWITCH constant in util.rs

* fmt

* refactor: implement Default trait for CefApiHandler

* docs: add documentation for CefApiHandler and its JavaScript API functions

---------

Co-authored-by: not-elm <elmgameinfo@gmail.com>

examples/extensions.rs

@@ -0,0 +1,71 @@
+//! Example demonstrating custom JavaScript extensions via CEF's register_extension.
+//!
+//! This example shows how to create global JavaScript APIs that are available
+//! in all webviews before any page scripts run.
+
+use bevy::prelude::*;
+use bevy_cef::prelude::*;
+use serde::Deserialize;
+
+fn main() {
+    App::new()
+        .add_plugins((
+            DefaultPlugins,
+            CefPlugin {
+                extensions: CefExtensions::new().add(
+                    "myGame",
+                    r#"
+                    var myGame = {
+                        version: "1.0.0",
+                        sendScore: function(score) {
+                            window.cef.emit('score_update', { score: score });
+                        }
+                    };
+                "#,
+                ),
+                ..Default::default()
+            },
+            JsEmitEventPlugin::<ScoreUpdate>::default(),
+        ))
+        .add_systems(
+            Startup,
+            (spawn_camera, spawn_directional_light, spawn_webview),
+        )
+        .add_observer(on_score_update)
+        .run();
+}
+
+#[derive(Deserialize, Debug)]
+struct ScoreUpdate {
+    score: u32,
+}
+
+fn on_score_update(trigger: On<Receive<ScoreUpdate>>) {
+    info!("Received score update: {:?}", trigger.score);
+}
+
+fn spawn_camera(mut commands: Commands) {
+    commands.spawn((
+        Camera3d::default(),
+        Transform::from_translation(Vec3::new(0., 0., 3.)).looking_at(Vec3::ZERO, Vec3::Y),
+    ));
+}
+
+fn spawn_directional_light(mut commands: Commands) {
+    commands.spawn((
+        DirectionalLight::default(),
+        Transform::from_translation(Vec3::new(1., 1., 1.)).looking_at(Vec3::ZERO, Vec3::Y),
+    ));
+}
+
+fn spawn_webview(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<WebviewExtendStandardMaterial>>,
+) {
+    commands.spawn((
+        CefWebviewUri::local("extensions.html"),
+        Mesh3d(meshes.add(Plane3d::new(Vec3::Z, Vec2::ONE))),
+        MeshMaterial3d(materials.add(WebviewExtendStandardMaterial::default())),
+    ));
+}