[prefers-color-scheme] release major version

[romainmenke]

• [prefers-color-scheme] remove old CDN urls #441
• [prefers-color-scheme] add browser tests #444
• [prefers-color-scheme] check spec compliance #448
• [prefers-color-scheme] housekeeping #450
• migration docs

[Antonio-Laguna]

This can now be closed as this has been released!