[prefers-color-scheme] add browser tests

[romainmenke]

[romainmenke]

These tests are minimal but cover these cases :

• native :
  • no preference (e.g. light)
  • light
  • dark
• polyfill :
  • no preference (e.g. light)
  • light
  • dark
• polyfill after toggle :
  • no preference (e.g. light)
  • light
  • dark

https://github.com/csstools/postcss-plugins/blob/prefers-color-scheme--compassionate-alpine-dachsbracke-f5f77b6255/plugins/css-prefers-color-scheme/test/_browser.html