re-implement the eslint `prefer-const` rule

### Description

the `prefer-const` rule included by default in eslint does not understand reactive statements with curly braces:
```svelte
<script lang="ts">
    let foo: number
    $: {
        foo = 1 // 'foo' is never reassigned. Use 'const' instead. (prefer-const)
    }
</script>

{foo}
```