docs: add library dartdoc and 100% public_member_api_docs coverage

Author: mfazrinizar

lib/flutter_secure_dotenv_generator.dart

@@ -1,3 +1,9 @@
+/// Code generator for [flutter_secure_dotenv](https://pub.dev/packages/flutter_secure_dotenv).
+///
+/// Reads `.env` files at build time, encrypts the values with AES-CBC, and
+/// emits a `part` file containing the encrypted data.
+library;
+
 import 'package:build/build.dart';
 import 'package:source_gen/source_gen.dart';
 
@@ -7,7 +13,6 @@ import 'src/annotation_generator.dart';
 ///
 /// Takes [options] as a parameter to configure the builder.
 Builder flutterSecureDotEnvAnnotation(BuilderOptions options) =>
-    SharedPartBuilder(
-      [FlutterSecureDotEnvAnnotationGenerator(options)],
-      'flutter_secure_dot_env_annotation',
-    );
+    SharedPartBuilder([
+      FlutterSecureDotEnvAnnotationGenerator(options),
+    ], 'flutter_secure_dot_env_annotation');