append a linebreak when serializing non-entry elements

[grafi-tt]

Hi,

With current implementation, when we parse and serialize a bib

@string{ foo = "Foo" }
@string{ bar = "Bar" }

then the following is obtained.

@string{ foo = "Foo" }@string{ bar = "Bar" }

I suppose this behaviour is undesirable for most people.

[grafi-tt]

I checked the CI error. Seemingly it is not related to my change.

[inukshuk]

Looks good, thank you!