Add canSend function to check mail sending capability

[tgloc]

Description:

This pull request introduces a new function named canSend to the flutter_email_sender package. This function helps developers determine if the device is currently capable of sending emails.

Motivation:

On iOS devices, users need to have a default email app installed (e.g., Apple Mail) to send emails through the flutter_email_sender package. Without an email app, sending emails will fail. This new function provides a way to check compatibility before attempting to send an email, improving the user experience.

Implementation:

• A new function named canSend() is added to the FlutterEmailSender class.
• This function utilizes platform-specific checks to determine if a compatible email app is installed.
  • On iOS, it can potentially leverage a framework like MessageUI to check for available mail services.
  • On Android, the logic might involve checking for installed email apps or mail intent capabilities.
• The function should return a bool value indicating whether the device can send emails.

Example Usage:

import 'package:flutter_email_sender/flutter_email_sender.dart';

Future<void> sendEmail() async {
  if (await FlutterEmailSender.canSend()) {
    // Proceed with email sending logic
    final email = Email(
      senderName: "Your Name",
      senderAddress: "your_email@example.com",
      recipients: ['recipient1@example.com', 'recipient2@example.com'],
      subject: 'Your Email Subject',
      body: 'Your Email Body',
    );

    await email.send();
    
  } else {
    // Inform the user that email cannot be sent on this device
    print("This device cannot send emails. Please install an email app.");
  }
}

Feedback:

We welcome any feedback or suggestions on this pull request. Please feel free to leave comments or ask questions.