Increase transclusion line break spacing

[tyler-dot-earth]

Makes line breaks in transclusions have a spacing more similar to normal line breaks.

Before:

After:

While a little opinionated, it feels more right to me - especially given that you can't really just do 2 newlines as that would then break the ability to transclude since that's how ^block-ids in markdown work.

Try it out and let me know your thoughts. I've been using it for a while, it works really well for me.

[birneee]

@tyler-dot-earth thank you for you pull request

but I think this is not related to adwaita