[12.x] Add dd() to Mailable

[braxey]

This mirrors the behavior of returning a Mailable from a controller or route, but not all mail is sent from controllers.

Developers can now preview mail directly from any context, like services, without sending it.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!