Implement conversions for 666 colors

[bugadani]

Thank you for helping out with embedded-graphics development! Please:

• Check that you've added passing tests and documentation
• Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, etc) if your changes affect the public API
• Run rustfmt on the project
• Run just build (Linux/macOS only) and make sure it passes. If you use Windows, check that CI passes once you've opened the PR.

PR description

This makes CSS colors available for Rgb666 and Bgr666

[jamwaffles]

Good catch, thanks!