Support nested elements on @child and @children

**I'm submitting a feature request**

**Current behavior:**

`@children` and `@child` select immediate children only.

* **What is the expected behavior?**

`@children` and `@child` should select arbitrary nested children.

* **What is the motivation / use case for changing the behavior?**

This is a great and important feature, and it is a shame that it boxes you into structuring your markup a particular way.

If the reason for limiting to immediate children was performance, this could be configurable:

```javascript
@children('my-element') a; // immediate children
@children({ selector: 'my-element', recursive: true }) b; // all children
```

Start by locating the implementations and tests for @child and @children, then trace how they currently limit selection to immediate children. Define and verify the expected behavior for arbitrary nesting while preserving existing immediate-child behavior, including whether recursive selection should be configurable as suggested.

Written by the indexing model from the issue text.