Extensions aren't included in the intellisense

Example:
```
@tailwind base;
@tailwind components;
@tailwind utilities;

.flex-center {
@apply flex justify-center items-center;
}
```

The issue provides a Tailwind CSS example using @apply and asks for extensions to be included in IntelliSense, but names no files, tests, or entry points. Begin by locating the IntelliSense implementation and its handling of @apply; done means the shown utility extensions appear in IntelliSense, with coverage for this example.

索引モデルが issue の本文から書いたものです。