[11.x] Add "head" slot to email layout

[hivokas]

Currently if I want to add a few lines of code to the head of email layout, I have to publish the entire file. But I'd prefer not to do it to keep the repository cleaner and to keep the email layout file up-to-date with the framework.

This PR adds a head slot which allows me to easily add a few lines of code to the head of email layout, like this:

<x-slot:head>
<style>
@media only screen and (max-width: 600px) {
.inner-body {
border-radius: 0 !important;
}
}
</style>
</x-slot:head>

If you find this addition useful, it would be cool to see it merged. If not, it's fine too because I can always publish the entire email layout file, even though I'd prefer not to.