refactor(example): restructure example project folders (#157)

- Split into basic_example, nullable_example, constant_example, and multi_env_example
- Split multi_env_example into merge_in_one_file and separated_files

Author: femrek

examples/envied_example/lib/env.dart renamed to examples/envied_example/lib/basic_example/env.dart

@@ -1,5 +1,5 @@
-import 'package:example/env.dart';
-import 'package:example/envs.env.dart';
+import 'package:example/basic_example/env.dart';
+import 'package:example/multi_env_example/merge_in_one_file/envs.env.dart';
 
 void run() {
   print(Env.key1);

examples/envied_example/lib/env.g.dart renamed to examples/envied_example/lib/basic_example/env.g.dart

@@ -1,9 +1,9 @@
 // lib/env.dart
+import 'package:example/multi_env_example/app_env_fields.dart';
+
 import 'package:envied/envied.dart';
 import 'package:example/example_enum.dart';
 
-import 'app_env_fields.dart';
-
 part 'envs.env.g.dart';
 
 @Envied(

examples/envied_example/lib/constant_case_env.dart renamed to examples/envied_example/lib/constant_example/constant_case_env.dart

@@ -1,4 +1,5 @@
-import 'app_env_fields.dart';
+import 'package:example/multi_env_example/app_env_fields.dart';
+
 import 'debug_env.dart';
 import 'release_env.dart';
 

examples/envied_example/lib/constant_case_env.g.dart renamed to examples/envied_example/lib/constant_example/constant_case_env.g.dart

@@ -1,8 +1,8 @@
 import 'package:envied/envied.dart';
 import 'package:example/example_enum.dart';
+import 'package:example/multi_env_example/app_env_fields.dart';
 
 import 'app_env.dart';
-import 'app_env_fields.dart';
 
 part 'debug_env.g.dart';