added both json ad text format for task exporting

Author: Pavel401

lib/views/profile/profile.dart

@@ -78,9 +78,37 @@ class _ProfilePageState extends State<ProfilePage> {
                     .toIso8601String()
                     .replaceAll(RegExp(r'[-:]'), '')
                     .replaceAll(RegExp(r'\..*'), '');
-                exportTasks(
-                  contents: tasks,
-                  suggestedName: 'tasks-$now.txt',
+
+                showDialog(
+                  context: context,
+                  builder: (BuildContext context) {
+                    return AlertDialog(
+                      title: const Text("Export Format"),
+                      content: const Text("Choose the export format:"),
+                      actions: <Widget>[
+                        TextButton(
+                          child: const Text("JSON"),
+                          onPressed: () {
+                            Navigator.of(context).pop();
+                            exportTasks(
+                              contents: tasks,
+                              suggestedName: 'tasks-$now.json',
+                            );
+                          },
+                        ),
+                        TextButton(
+                          child: const Text("TXT"),
+                          onPressed: () {
+                            Navigator.of(context).pop();
+                            exportTasks(
+                              contents: tasks,
+                              suggestedName: 'tasks-$now.txt',
+                            );
+                          },
+                        ),
+                      ],
+                    );
+                  },
                 );
               },
               () {