Diagnostic formatter should highlight trivia in between consecutive highlight nodes

Currently, if a diagnostic has two nodes to highlight, it will highlight them individually, leaving the trivia in between them non-highlighted. The easiest way to see this is by running

```bash
swift-parser-cli print-diags -s 'for x = 1; x < 10; x++ {}'
```

Note that the space between `x` and `=` should also be highlighted.