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/cdp/testing.zig

@@ -161,7 +161,7 @@ const Page = struct {
     aux_data: []const u8 = "",
     doc: ?*parser.Document = null,
 
-    pub fn navigate(_: *Page, url: []const u8, aux_data: []const u8) !void {
+    pub fn navigate(_: *Page, url: URL, aux_data: []const u8) !void {
         _ = url;
         _ = aux_data;
     }