Audit components for compatibility with high-contrast mode

[jcfranco]

Background

Stems from #3875. To improve a11y, we should audit all components to make sure they are usable under high-contrast mode.

Desired Outcome

All components are compliant with high-contrast mode. Updating the conventions doc with common patterns is a plus.

[jcfranco]

@mpriour has volunteered some cycles to work on this. Thanks! 🙇

[mpriour]

@jcfranco @benelan Here is the high contrast audit I did.
https://github.com/Esri/calcite-components/wiki/High-Contrast-Audit

I'm happy to make any edits for clarification or consistency.