feat: HTML Support

### Is there an existing issue outlining your improvement?

- [x] I have searched the existing issues and my improvement has not been raised yet.

### What would you like to see added and/or changed?

This issue describes a future consideration that will need to be addressed when Typst stabilises HTML outputting.

Currently, we use `set page(...)` to modify page background colours. This will need to be altered in the future to check whether or not the [`target`](https://typst.app/docs/reference/foundations/target/) is `HTML`.

Depending on how HTML outputting is utilised, this feature may not be implemented. For an example as to why, consider the use case of writing blog articles. As said in the documentation for [`elem`](https://typst.app/docs/reference/html/elem/):

> Typst's HTML export automatically generates the appropriate tags for most elements. However, sometimes, it is desirable to retain more control. For example, when using Typst to generate your blog, you could use this function to wrap each article in an `` tag.

For small use-cases, it makes little sense to modify this package, as users can manually set their theming as needed. If it becomes possible to make more advanced web documents, then it will might make sense to at least provide a means to perform basic styling.

No implementation file, test, or entry point is named. Start by reviewing Typst's HTML target and the current set page(...) usage once HTML output stabilises; done requires an agreed scope for HTML styling and a documented decision on whether this package should support it.

Written by the indexing model from the issue text.