Add MD3 medium and high contrast levels

**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