Communicate page navigation state via notifications

In order to support click handling on anchors from JavaScript, we need some hook
from the page/session to the CDP instance. This first phase adds notifications
in page.navigate, as well as a primitive notification hook to the session.

CDP's existing Page.navigate uses this new notifiation system.

src/notification.zig

@@ -0,0 +1,11 @@
+const URL = @import("url.zig").URL;
+
+pub const Notification = union(enum) {
+    page_navigate: PageEvent,
+    page_navigated: PageEvent,
+
+    pub const PageEvent = struct {
+        ts: u32,
+        url: *const URL,
+    };
+};