Fix parsing bug in SelectorParser (#20090)

While working on another feature, I noticed that a selector such as
`.foo::before` was parsed as:

```ts
[
  {
    kind: 'compound',
    nodes: [
      { kind: 'selector', value: '.foo' },
      { kind: 'selector', value: ':' },
      { kind: 'selector', value: '::before' },
    ],
  },
]
```

Instead of:
```ts
[
  {
    kind: 'compound',
    nodes: [
      { kind: 'selector', value: '.foo' },
      { kind: 'selector', value: '::before' },
    ],
  },
]
```

So far this hasn't been a real issue in practice, but it is in a
follow-up PR that I'm working on. To keep things separated, I wanted to
fix this behavior in a dedicated PR instead.

## Test plan

1. Added a test case for this situation
2. Other tests still pass