This PR adds support for `HTML` href elements. It also refactored the
way we stored the regions, this was done because otherwise I had to add
2 extra arguments to each `HTML` parser method. It's now also more
inline with how we have done it for the highlights.

**Small note**: the markdown parser only supports HTML href tags inside
a paragraph tag. So adding them as a root node will result in just
showing the inner text. This is a limitation of the markdown parser we
use itself.

**Before**
<img width="935" height="174" alt="Screenshot 2025-11-08 at 15 40 28"
src="https://github.com/user-attachments/assets/42172222-ed49-4a4b-8957-a46330e54c69"
/>

**After**
<img width="1026" height="180" alt="Screenshot 2025-11-08 at 15 29 55"
src="https://github.com/user-attachments/assets/9e139c2d-d43a-4952-8d1f-15eb92966241"
/>

**Example code**
```markdown
<p>asd <a href="https://example.com">Link Text</a> more text</p>
<p><a href="https://example.com">Link Text</a></p>

[Duck Duck Go](https://duckduckgo.com)
```

**TODO**:
- [x] Add tests

cc @bennetbo

Release Notes:

- Markdown Preview: Add support for `HTML` href elements.

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>