Do not emit @keyframes in @theme reference (#16120)

This PR fixes na issue where `@keyframes` were emitted if they wre in a
`@theme
reference` and anothe `@theme {}` (that is not a reference) was present.

E.g.:

```css
@reference "tailwindcss";

@theme {
  /* ... */
}
```

Produces:
```css
:root, :host {
}
@keyframes spin {
  to {
    transform: rotate(360deg);
  }
}
@keyframes ping {
  75%, 100% {
    transform: scale(2);
    opacity: 0;
  }
}
@keyframes pulse {
  50% {
    opacity: 0.5;
  }
}
@keyframes bounce {
  0%, 100% {
    transform: translateY(-25%);
    animation-timing-function: cubic-bezier(0.8, 0, 1, 1);
  }
  50% {
    transform: none;
    animation-timing-function: cubic-bezier(0, 0, 0.2, 1);
  }
}
```

With this PR, the produced CSS looks like this instead:
```css
:root, :host {
}
```

Note: the empty `:root, :host` will be solved in a subsequent PR.

### Test plan

Added some unit tests, and a dedicated integration test.

integrations/cli/index.test.ts

@@ -1196,3 +1196,65 @@ test(
     `)
   },
 )
+
+test(
+  '@theme reference should never emit values',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'src/index.css': css`
+        @reference "tailwindcss";
+
+        .keep-me {
+          color: red;
+        }
+      `,
+    },
+  },
+  async ({ fs, spawn, expect }) => {
+    let process = await spawn(
+      `pnpm tailwindcss --input src/index.css --output dist/out.css --watch`,
+    )
+    await process.onStderr((m) => m.includes('Done in'))
+
+    expect(await fs.dumpFiles('./dist/*.css')).toMatchInlineSnapshot(`
+      "
+      --- ./dist/out.css ---
+      .keep-me {
+        color: red;
+      }
+      "
+    `)
+
+    await fs.write(
+      './src/index.css',
+      css`
+        @reference "tailwindcss";
+
+        /* Not a reference! */
+        @theme {
+          --color-pink: pink;
+        }
+
+        .keep-me {
+          color: red;
+        }
+      `,
+    )
+    expect(await fs.dumpFiles('./dist/*.css')).toMatchInlineSnapshot(`
+      "
+      --- ./dist/out.css ---
+      .keep-me {
+        color: red;
+      }
+      "
+    `)
+  },
+)