re-implement the eslint prefer-const rule

[DetachHead]

Description

the prefer-const rule included by default in eslint does not understand reactive statements with curly braces:

<script lang="ts">
    let foo: number
    $: {
        foo = 1 // 'foo' is never reassigned. Use 'const' instead. (prefer-const)
    }
</script>

{foo}

[ota-meshi]

Thank you for the bug report. I think it's probably a parser bug.