refactor: facelift both Envied and EnviedGenerator to Dart 3.0

examples/envied_example/.env

@@ -1,2 +1,5 @@
-KEY1=VALUE1
-KEY2=VALUE2
+KEY1=foo
+KEY2=bar
+key3=baz
+key4=123
+key5=false

examples/envied_example/.env_debug

@@ -1,2 +1,5 @@
-KEY1=DEBUG1
-KEY2=DEBUG2
+KEY1=debug_foo
+KEY2=debug_bar
+key3=debug_baz
+key4=456
+key5=true

examples/envied_example/lib/app_env.dart

@@ -2,7 +2,7 @@ import 'app_env_fields.dart';
 import 'debug_env.dart';
 import 'release_env.dart';
 
-abstract class AppEnv implements AppEnvFields {
+abstract interface class AppEnv implements AppEnvFields {
   /// NOTE: This is here just as an example!
   ///
   /// In a Flutter app you would normally import this like so

examples/envied_example/lib/app_env_fields.dart

@@ -1,5 +1,5 @@
 /// Both DebugEnv and ReleaseEnv must implement all these values
-abstract class AppEnvFields {
+abstract interface class AppEnvFields {
   abstract final String key1;
   abstract final String key2;
   abstract final String key3;

examples/envied_example/lib/debug_env.dart

@@ -6,7 +6,7 @@ import 'app_env_fields.dart';
 part 'debug_env.g.dart';
 
 @Envied(name: 'Env', path: '.env_debug')
-class DebugEnv implements AppEnv, AppEnvFields {
+final class DebugEnv implements AppEnv, AppEnvFields {
   const DebugEnv();
 
   @override

examples/envied_example/lib/env.dart

@@ -4,11 +4,11 @@ import 'package:envied/envied.dart';
 part 'env.g.dart';
 
 @Envied(path: '.env')
-abstract class Env {
+final class Env {
   @EnviedField(varName: 'KEY1')
-  static const key1 = _Env.key1;
+  static const String key1 = _Env.key1;
   @EnviedField(varName: 'KEY2')
-  static const key2 = _Env.key2;
+  static const String key2 = _Env.key2;
   @EnviedField()
   static const String key3 = _Env.key3;
   @EnviedField()

examples/envied_example/lib/release_env.dart

@@ -6,7 +6,7 @@ import 'app_env_fields.dart';
 part 'release_env.g.dart';
 
 @Envied(name: 'Env', path: '.env')
-class ReleaseEnv implements AppEnv, AppEnvFields {
+final class ReleaseEnv implements AppEnv, AppEnvFields {
   const ReleaseEnv();
 
   @override