Add support for alternate StyleSheets (#5168)
Add support for alternate StyleSheets (#5168)

- Test page: https://media.prod.mdn.mozit.cloud/samples/cssref/altstyles/index.html
- Official spec about this behavior: https://html.spec.whatwg.org/#rel-alternate
- https://developer.mozilla.org/en-US/docs/Web/CSS/Alternative_style_sheets
- When title is specified it will be registered for Firefox to only load it when it has been selected under the subMenu (View > Page-View).
- Copying the title attribute to the `--sync` element will follow the alternate stylesheet.
- Resolves #737
darkreader/darkreader,darkreader/darkreader,alexanderby/darkreader,alexanderby/darkreader,darkreader/darkreader,alexanderby/darkreader