Make it possible for markdown mails to also accept template strings

[SuperDJ]

Something similar as #40425 or #40553 would be nice to have in markdown mails.

In a project I'm working on I need to make mail "templates". These templates are basically markdown mails. The downside is that right now each mail template needs to be stored as a blade file. For me just being able to send the mail template as a string to $mail->markdown('template string') would be enough.