Minimal fix to preserve newlines

Fixes all forms that do not contain `vellum:ignore=“retain”` attributes.

Note that this is as good as the pre-RequireJS Vellum did. [vellum-before-requirejs formdesigner.ignoreButRetain.js](https://github.com/dimagi/Vellum/blob/abb5c227d452d244ec3f7072160fee84054a8d45/src/js/formdesigner.ignoreButRetain.js#L52-L80) did the same parse/serialize dance if there were ignored nodes, which would presumably cause newlines, etc. to be lost on the next load/save.

Beginning of a more complete fix: https://github.com/dimagi/Vellum/tree/133665-missing-newlines-continued