Add semantic slots, starting with text fields

[phkuo]

Pull request checklist

• Addresses an existing issue: #0000
• Include a change request file if publishing
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Description of changes

Lay some groundwork for converting to semantic slots, starting with input text fields.

Focus areas to test

(optional)

[micahgodbolt]

Looks fine to me (assuming no visual regressions). I've seen this approach before from @phkuo and it closely mirrors how I'd do it with external Theo tokens. I'm mostly curious how this would look using Glamor, though I don't feel anything here would make converting to Glamor harder.

Maybe the bigger question is how close are we to converting to Glamor (pretty close I think), and should this work just be done in coordination with that glamor conversion.

[micahgodbolt]

@dzearing how are you feeling about this approach and what it'd look like with Glamor? I don't see any problem creating something similar, but wanted to get your feedback before we merge.

And @phkuo, just an FYI that you have some conflicts to resolve.

[dzearing]

is a radio button any different from a checkbox?

[phkuo]

Unfortunately yes. For example, in the selected state, it uses a blue background and a white checkmark, whereas radio buttons have a white background and a blue dot.

[phkuo]

Doing design review before completing this PR.
@betrue-final-final

[phkuo]

@dzearing @micahgodbolt Could I get this reviewed and signed off please?

[micahgodbolt]

@phkuo a few conflicts to resolve in textfield