feat: add createViewerWithOptions to ThermionFlutterPlugin and mark createViewer as deprecated

thermion_flutter/thermion_flutter/lib/thermion/thermion_flutter_plugin.dart

@@ -60,6 +60,7 @@ class ThermionFlutterPlugin {
     }
   }
 
+  @Deprecated("Use createViewerWithOptions")
   static Future<ThermionViewer> createViewer({String? uberArchivePath}) async {
     if (_initializing) {
       throw Exception("Existing call to createViewer has not completed.");

thermion_flutter/thermion_flutter_platform_interface/lib/thermion_flutter_platform_interface.dart

@@ -4,7 +4,7 @@ import 'package:thermion_dart/thermion_dart/thermion_viewer.dart';
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 import 'thermion_flutter_texture.dart';
 
-abstract class ThermionFlutterOptions {
+class ThermionFlutterOptions {
   final String? uberarchivePath;
 
   ThermionFlutterOptions({this.uberarchivePath});