## What This PR Does
This PR adds support for HTML alignment styles to be applied to
Paragraph and Heading elements and their children. Here is what this
looks like before vs after this PR (both images use the same markdown
below):

```markdown
<p style="text-align: center;">
  <a target="_blank" href="https://github.com/"><img width="150" height="150" src="https://upload.wikimedia.org/wikipedia/commons/c/c2/GitHub_Invertocat_Logo.svg"></a>
</p>
```

**BEFORE:**

<img width="742" height="242" alt="image"
src="https://github.com/user-attachments/assets/4ca8b8d9-0606-45f5-8a0e-cafaaac47d97"
/>

**AFTER:**

<img width="1274" height="267" alt="image"
src="https://github.com/user-attachments/assets/2c347ce7-75b9-4ef6-9598-b1eda7272ef5"
/>

## Notes
I used `style="text-align: center|left|right;"` instead of
`align="center|right|left"` since `align` has been [deprecated in
HTML5](https://www.w3.org/TR/2011/WD-html5-author-20110809/obsolete.html)
for block-level elements. The issue this PR solves mentioned that github
supports the `align="center|right|left"` attribute, so I'm unsure if the
Zed team would want to have parity there. Feel free to let me know if
that would be something that should be added, however for now I've
decided to follow the HTML5 standard.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes https://github.com/zed-industries/zed/issues/51062

Release Notes:

- Fixed HTML alignment styles not being applied in markdown previews

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>