[9.x] Add raw content property for mailables

[Xammie]

This is a follow up from #44696 (comment).

This PR introduces a new property and method on the mailable content class to supply a raw html string.

public function content() {
    return new Content(
        raw: 'raw html string'
    );
}

The classic mailable approach already has this feature.

public function build() {
    return $this->html('raw html string');
}

[driesvints]

Hi there. We fixed the failing tests on 9.x which are causing this PR to fail as well. Can you please rebase your PR with 9.x and mark this PR as ready for review when all tests are passing again? Thanks

[ascentcreative]

Are there plans for a rawText() method at all?

I'm using soundasleep/html2Text to convert my HTML view to text-only, and it would be great to be able to pass that value into the Content. At the moment I can fudge it by passing the text via a blade, but it's a bit clunky...