[feature] Email module

Thank you all for participating in the Email third part provider poll.
Our email implementation is in progress, but not the highest priority compared to the 4.0.0 release.

Planned so far:
[x] separate Email\Email class, which encapsulates an Email entity
[x] Email\TransporterInterface, which abstracts the behavior of a mail transport agent, or a wrapper for same, or even an external email service
[.] Email\Handlers\BaseHandler, which implements the common methods of a Transporter
[.] Email\Handlers\MailHandler, which uses (PHP)mail to send emails; was in CI3
[.] Email\Handlers\SendmailHandler, which uses sendmail to send emails; was in CI3
[.] Email\Handlers\SMTPHandler, which uses SMTP to send emails; was in CI3
[.] Email\Handlers\PHPMailer, adapter for 3rd party module suggested by poll
[.] revised user guide writeup for handling email
[x] unit testing for Email\Email
[.] unit testing for Email\Handlers\...
[ ] using an external email testing service for unit testing; leaning towads https://mailcatcher.me/

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This is a roadmap feature, or "epic", and a card on the roadmap board.
Component tasks will show up as issues or PRs on the development board.
We welcome comments & suggestions below.

PHPMailer looks like an important choice for the CodeIgniter community, and a complete solution.

The big difference between PHPMailer and the in-progress Email module for CodeIgniter4 is that we have separated the Email and the handling classes, unlike PHPMailer and the CI3 implementation, which bundle them together into a single class (with supporting classes).

We could just use PHPMailer and be done with it, or we can continue on the current path, which would allow the use of a number of mailers with minimal code change.

What do you think?