Prevent duplicate suggestions when using @theme and @utility together

[thecrypticace]

Fixes tailwindlabs/tailwindcss-intellisense#1313

Right now given this CSS:

@theme reference {
  --text-header: 1.5rem;
}

@utility text-header {
  text-transform: uppercase;
}

You'll see two entries for text-header in IntelliSense completions but we only want you to see one. This PR solves this by merging their modifier lists and de-duping by class name.