It’s 2019, and users of JetBrains apps now have automatic soft-wraps for Markdown documents – rejoice! 🎉🎊🥳
Alas it is not enabled by default, so you’ll need to visit your preferences and check a box: On WebStorm 2019.2 at least, navigate to Editor > General > Soft Wraps and click your way to Markdown-soft-wrap bliss.