[go_router_builder] Support extension types (#9458)

fix flutter/flutter#170870

Extended `type_helpers.dart` to support [Dart extension types](https://dart.dev/language/extension-types).

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: koji-1009

packages/go_router_builder/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 3.2.0
+
+- Adds support for`extension type`.
+
 ## 3.1.0
 
 - Updates dependencies to use the latest `analyzer`, `build`, and `source_gen`.

packages/go_router_builder/README.md

@@ -334,7 +334,7 @@ class RedirectRoute extends GoRouteData {
 
 ## Type conversions
 
-The code generator can convert simple types like `int` and `enum` to/from the
+The code generator can convert simple types like `int`, `enum`, and `extension type` to/from the
 `String` type of the underlying pathParameters:
 
 <?code-excerpt "example/lib/readme_excerpts.dart (BookKind)"?>