Comments in multi language files

[DoctorRyner]

In such files as svelte we need to properly handle comments, e.g. :toggle_comments should work properly in proper places.

For Svelte when I put a comment it display syntax errors

<script>
  // ..
</script>

<!-- ... -->

<style>
  // ...
</style>

[the-mikedavis]

Duplicate of #7364