No Autocompletion for Stylus in vscode

[Pytal]

Describe the bug

Autocompletion does not work when writing styles with stylus in a sapper project.

To Reproduce Steps to reproduce the behavior:

1. Scaffold a sapper project and install stylus

   npm add -D stylus

2. Setup svelte-preprocess in svelte.config.js

   
   const sveltePreprocess = require('svelte-preprocess')

module.exports = { preprocess: sveltePreprocess() }

3. Install the [Svelte for VS Code](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode) and [language-stylus](https://marketplace.visualstudio.com/items?itemName=sysoev.language-stylus) extensions

4. Write styles and trigger completion suggestions
```svelte
<style lang='stylus'>
  main {
    display: flex;
  }
</style>

Expected behavior

Autocompletion suggestions for CSS properties should appear just as when writing css or scss.

System (please complete the following information):

• OS: Windows
• IDE: VSCode
• Plugin/Package: Svelte for VSCode

[Monkatraz]

I did a quick-check as I thought my PR https://github.com/sveltejs/language-tools/pull/657 would've fixed this (I added source.stylus to the extension's package.json as it was missing) and yet it appears it hasn't. Normally, this should be all you need to do for autocomplete to work in an embedded language, or at least I thought so? Taking an educated guess, I bet the issue is with the Stylus extension itself not working within embedded blocks for whatever reason.

[dummdidumm]

source.stylus is only related to the highlighting in this context, the autocompletion has to be implemented separately through a language server/client.

[Monkatraz]

Oh, to be clear, what I mean is that the field mapping the source.stylus scope to the stylus language was missing in the package.json. What I had gathered from the VSCode docs is that you needed to assign some scope to an embedded language to get autocomplete to work. However, beyond that, I have no idea what I'm talking about and I was just documenting that my fix didn't work or was unrelated.

[jeremyjacob]

I have played around with this to no avail. If someone has a solution please share!