Support custom components which get applied to the HTML AST.

[emmalexandria]

When a Rehype plugin is used, the elements it affects/creates don't seem to have the relevant custom components applied. As far as I can tell, this is because custom components are generated from the markdown AST. It would be great if they could rather be generated from the HTML AST, with properties on the HTML tags being passed through to the custom Svelte components. I do understand this has its issues, especially with plugins like rehype-toc. Maybe the list of custom components could be split into those that apply to the Markdown AST and those that apply to the HTML AST?