Add support for @const

[catapop84]

https://svelte.dev/docs#template-syntax-const

<script>
    export let boxes;
</script>

{#each boxes as box}
    {@const area = box.width * box.height}
    {box.width} * {box.height} = {area}
{/each}

in this example I receive this error: Expected html or debug

I'm using plugin version 0.22.1 with webstorm 2022.1.4

[dominikg]

duplicate of #268