`$css` doesn't work with css-nesting

[Vulcagon]

<script>
    import MyComponent from 'path/to/MyComponent.svelte';
</script>

<MyComponent class={$css('my-class')} />

<style>
    /* Unused CSS selector ".my-class" */
    .my-class {
        ...

        &:hover {
            ...
        }
    }
</style>

It works if you don't use css-nesting

<script>
    import MyComponent from 'path/to/MyComponent.svelte';
</script>

<MyComponent class={$css('my-class')} />

<style>
    /* No `Unused CSS selector` warning */
    .my-class {
        ...
    }

    .my-class:hover {
        ...
    }
</style>

[JanNitschke]

Nesting is currently unsupported. Implementing validation for this requires a lot more parsing work. If you want to use nesting please use a preprocessor like scss for now.

Also added a note to the readme