By default, MediaWiki ignores single line breaks, treating them as a single space. To insert a new line in a page, you must insert two consecutive line breaks in the source, which results in a new paragraph: </p><p>. It's also possible to insert <br /> directly into the source.
How can I get MediaWiki (control its parsing engine) to convert single line breaks in the source to <br /> elements in the resulting HTML?
Use https://www.mediawiki.org/wiki/Extension:Poem
<poem>
Roses are red
Violets are blue
</poem>