Allow matching blocks before or after a given block

### Description

Inspired by the internal use case given here: https://l.alley.dev/7682a06198

The `before` and `after` parameters might accept an array of `match_block()` parameters, like

```
'before' => [ 'name' => 'my-plugin/my-custom-block' ],
```

```
'after' => [ 'with_innerhtml' => 'Get the whole story' ]
```

### Use Case

As a developer, I want to be able to match the blocks in post content before or after a given block so that I can deliver the two halves of content in separate properties to a third-party application that composes the results in its own way.