start implement generator code

cherrypick_generator/lib/inject_generator.dart

@@ -0,0 +1,38 @@
+import 'dart:async';
+import 'package:build/build.dart';
+import 'package:source_gen/source_gen.dart';
+import 'package:analyzer/dart/element/element.dart';
+import 'package:cherrypick_annotations/cherrypick_annotations.dart';
+
+class InjectGenerator extends GeneratorForAnnotation<Injectable> {
+  @override
+  FutureOr<String> generateForAnnotatedElement(
+    Element element,
+    ConstantReader annotation,
+    BuildStep buildStep,
+  ) {
+    print('[TRACE] Processing element: ${element.name}');
+
+    if (element is! FieldElement) {
+      throw InvalidGenerationSourceError(
+        'Inject can only be used on fields.',
+        element: element,
+      );
+    }
+
+    print('[TRACE] Starting code generation for element: ${element.name}');
+
+    final className = element.enclosingElement.name;
+    final fieldName = element.name;
+    final fieldType = element.type.getDisplayString(withNullability: false);
+    final annotationName = annotation.read('named').stringValue;
+
+    return '''
+extension \$${className}Inject on $className {
+  void init$fieldName() {
+    print("Injected $fieldType named '$annotationName' into $fieldName");
+  }
+}
+''';
+  }
+}