Feature request: support for the `source.css.postcss` scope

Hi @jonathantneal. Thank you for your work on this extension.

I'm using the [Svelte VS Code extension](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode) which adds the `source.css.postcss` scope to styles contained in a `` tag. I'm also using `postcss-preset-env` and I'd like to use your extension for syntax highlighting.

Unfortunately, your extension contributes to the `source.css` scope and not `source.css.postcss`. The latter scope is also used by the popular [Vetur extension](https://marketplace.visualstudio.com/items?itemName=octref.vetur), so I believe it'd be a good idea to update your extension to add support for both scopes.

I tried but couldn't figure out how to do this without either dropping support for the `source.css` scope (which I assume you want to maintain) or duplicating the grammar definition file for the other scope. I'm hoping you would consider addressing this yourself. You can read more about this [here](https://github.com/sveltejs/language-tools/issues/996).

Thank you!