In the CSS specification, `<container-query>` is now optional.

Old: `[ <container-name> ]? <container-query>`
New：`[ <container-name>? <container-query>? ]!`

This means that previously, to determine whether an element had container queries enabled,
we had to write it like this:

```css
@container foo (width >= 0) {
  .inner {
    color: green
  }
}
```

It can now be simplified to:

```css
@container foo {
  .inner {
    color: green
  }
}
```

- Spec: w3c/csswg-drafts#11172
- Chrome implemented in: https://chromium-review.googlesource.com/c/chromium/src/+/7378819
- Safari implemented in: https://bugs.webkit.org/show_bug.cgi?id=302433

----

This PR also improves error messages; `@container foo () {}` now provides specific syntax error reasons:

```
The brackets cannot be empty
```