Invalid selector when nesting a selector beginning with a combinator

[Rich-Harris]

Describe the bug

This is valid CSS, but Svelte throws an 'Invalid Selector' error

.foo {
  > .bar {
    color: red;
  }
}

The same goes for other combinators

Reproduction

link

[Rich-Harris]

(note: it's easily worked around by adding an explicit & selector, i.e. & > .bar)