[Selection] SelectableRegion should support rich text clipboard

[chunhtai]

SelectableRegion currently only support PlainText data type. We should make sure we can store RichText data type.

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: SelectionArea(
        child: Scaffold(
          appBar: AppBar(title: const Text(_title)),
          body: Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: const <Widget>[
                Text('Row 1', style: TextStyle(fontSize: 30, color: Colors.red)),
                Text('Row 2'),
                Text('Row 3'),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

1. try to select and copy test with different style
2. paste into google doc or some richtext editor
3. currently it store as plainText.

Ideally it should preserve some rich text content.