[8.x] Change Visibility of the Markdown property in Mailable

[JamesFreeman]

I wanted to compare the output of a mailable against a stub, after some searching I found this as a solution:

Mail::assertSent(SomeMail::class, function (SomeMail $mail) {
    $mail = $mail->build();

    $this->assertStringEqualsFile(
        $this->removeTabsFromString(base_path('tests/Unit/Stubs/SomeMail.txt')),
        view($mail->view, $mail->buildViewData())
    );

    return true;
});

I wanted to do something similar for a markdown email, but I can't get access to this property as it's protected.

[driesvints]

This is a breaking change. You could maybe add a getter instead?

[JamesFreeman]

Sorry, I didn't think loosening property visibility would be a breaking change.

I'm happy to add a getter to be on the safe side, do you think it would be worth adding one for the view for consistency?

[driesvints]

@taylorotwell will need to decide on that

[taylorotwell]

I think the likelihood of this breaking an application is pretty small.