Add web startup analyzer to material 3 demo (#2144)

This adds a tool to measure web app startup for the Material 3 demo.

Demo:
- [Example app](https://flutter-web-perf-experiments.web.app/)
- [Material
3](https://flutter-web-perf-experiments--material3-vswzldcy.web.app/)
(open console)

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>
Co-authored-by: Kevin Moore <kevmoo@google.com>

web/_packages/web_startup_analyzer/pubspec.yaml

@@ -0,0 +1,19 @@
+name: web_startup_analyzer
+description: "Captures web startup timing data in a Flutter web app"
+version: 0.1.0-wip
+
+environment:
+  sdk: ^3.2.0
+  flutter: ^3.16.0
+
+dependencies:
+  flutter:
+    sdk: flutter
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^3.0.0
+flutter:
+  assets:
+    - lib/web_startup_analyzer.js