Reorganize v8 contexts and scope

- Pages within the same session have proper isolation
  - they have their own window
  - they have their own SessionState
  - they have their own v8.Context

- Move inspector to CDP browser context
  - Browser now knows nothing about the inspector

- Use notification to emit a context-created message
  - This is still a bit hacky, but again, it decouples browser from CDP

src/main.zig

@@ -100,7 +100,7 @@ pub fn main() !void {
             var session = try browser.newSession({});
 
             // page
-            const page = try session.createPage(null);
+            const page = try session.createPage();
 
             _ = page.navigate(url, .{}) catch |err| switch (err) {
                 error.UnsupportedUriScheme, error.UriMissingHost => {