Add appearance-readonly to nimble-select and nimble-combobox

[mollykreis]

Pull Request

🤨 Rationale

Add appearance-readonly attribute to the nimble-select and nimble-combobox to style the components as readonly when they are disabled.

👩‍💻 Implementation

• Add appearance-readonly attribute (tied to appearanceReadOnly property) on the select and combobox
• Added styles to make disabled + appearanceReadOnly look like other readonly controls (neither select nor combobox has a readonly property or styles today)
• Updated storybook docs
• Updated matrix tests
  • As part of the matrix test updates, I split the select matrix story into 3 stories (one for each theme) because chromatic was timing out loading the story

🧪 Testing

• Manually tested
• Chromatic test

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.