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/cdp/testing.zig

@@ -122,7 +122,7 @@ const TestContext = struct {
         if (opts.html) |html| {
             parser.deinit();
             try parser.init();
-            const page = try bc.session.createPage(null);
+            const page = try bc.session.createPage();
             page.doc = (try Document.init(html)).doc;
         }
         return bc;