Add MD3 medium and high contrast levels #5092

Is your feature request related to a problem? Please describe.
The theme only generates the MD3 standard contrast level. MD3 defines three levels, and the higher two exist for accessibility: they raise the contrast between text and background so the UI stays readable for users with low vision and in bright light.

Describe the solution you'd like
Add the medium and high schemes for both light and dark, and a way to select the active level, with standard staying the default so existing apps are unaffected.

Additional context

• MD3 spec: https://m3.material.io/styles/color/system/overview