Support for preconnect link relationship for Google Fonts

[Google Fonts](https://fonts.google.com/specimen/Roboto) asks you to include two preconnect links when using one of their fonts. The link tags when copied from the Google Fonts site look like this:
```

```

The [rel attribute of links](https://github.com/axodotdev/axohtml/blob/d2c0379311357f0e9cec591eeeae480f17eec39a/typed-html/src/elements.rs#L72) is parsed with a [LinkType enum](https://github.com/axodotdev/axohtml/blob/d2c0379311357f0e9cec591eeeae480f17eec39a/typed-html/src/types/mod.rs#L186C20-L186C20) that is [missing a few valid values](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel). This [LinkType enum is also used for a tags](https://github.com/axodotdev/axohtml/blob/d2c0379311357f0e9cec591eeeae480f17eec39a/typed-html/src/elements.rs#L99) so a straight copy/paste into the enum might not be enough because not all types are supported in both tags!