Introduce more general notification capabilities

Replaces the existing, very specialized Notification with something more
general.

Currently, the existing page_navigate and page_navigated have been migrated.

Telemetry's page navigation event now also hooks into these events to generate
the telemetry record.

src/main.zig

@@ -97,7 +97,7 @@ pub fn main() !void {
             var browser = try Browser.init(app);
             defer browser.deinit();
 
-            var session = try browser.newSession({});
+            var session = try browser.newSession();
 
             // page
             const page = try session.createPage();