Custom Preprocessor
"Svelte's own parser understands only JavaScript, CSS and its HTML-like syntax. To make it possible to write components in other languages, such as TypeScript or SCSS, Svelte provides the preprocess API, which allows to easily transform the content of your markup and your style/script tags.
Writing your own preprocessor for, i.e SCSS is easy enough, but it can be cumbersome to have to always configure multiple preprocessors for the languages you'll be using.
svelte-preprocess is a custom svelte preprocessor that acts as a facilitator to use other languages with Svelte, providing multiple features, sensible defaults and a less noisy development experience."
Svelte Official
@sveltejs