Fix crash when formatting svelte files that have "let:class".

[chrsep]

Hi there, I encountered a crash when running the plugin on a svelte file that has a slot component like this:

  <p
    slot="test"
    let:class={className}
    class="h-full w-full p-2 backdrop-blur-0 {className}"
  />

Below was the error message and I've got a repro here:

I found that the plugin was trying to read the "value" attribute from the let directive, which is always undefined. So to avoid this, I added a check to make sure that the sorting only runs on the class attributes, not on a directive named class such as let:class={className}.

[bradlc]

Thank you!