Header elements (h1-h6) cannot be placed inside <summary>

The spec does say it's for Phrasing contents (which excludes header elements), but then allows them anyway.

> Content model:
> Phrasing content, optionally intermixed with heading content.

https://html.spec.whatwg.org/multipage/interactive-elements.html#the-summary-element

https://stackoverflow.com/questions/41227862/is-it-semantically-correct-to-use-h2-tag-inside-summary-tag

So the compile error for this code, does not follow the spec

```rust
let x: DOMTree = html! {

"test"

};
```

```rust
error[E0277]: the trait bound `h4<_>: axohtml::elements::PhrasingContent<_>` is not satisfied
--> src/main.rs:9:30
|
9 | let x: DOMTree = html! {
| ______________________________^
10 | |
11 | |

"test"

12 | |
13 | | };
| |_____^ the trait `axohtml::elements::PhrasingContent<_>` is not implemented for `h4<_>`
|
```

This feels gnarly to carve out an exception in our type system, so I understand if this isn't high priority. It's not high priority for me either