Add Color.toCssHexString and unit tests

[ErixenCruz]

Added Color.toCssHexString for getting the CSS hex string equivalent for a color.

[cesium-concierge]

Thank you so much for the pull request @ErixenCruz! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

• ❌ Missing CONTRIBUTORS.md entry.
  • Please add yourself to the contributors file!
• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[mramato]

What's the use case for this? toCssColorString can be used in CSS anywhere toCssHexString could also be used, right? And it supports alpha.

[mramato]

Also, hex strings with alpha work everywhere but IE11, so I'm not sure why we would leave alpha off: https://caniuse.com/#feat=css-rrggbbaa

[hpinkos]

I had a use case for this. There was a 3rd party component i wanted to send a color value to that only accepted hex strings

[hpinkos]

+1 for using #RRGGBBAA if alpha < 1 though, instead of just ignoring it

[ErixenCruz]

@hpinkos Updated with your feedback.

[hpinkos]

This should go under the Additions section instead of the Breaking Changes section

[hpinkos]

@ErixenCruz Be sure to add yourself to https://github.com/CesiumGS/cesium/blob/master/CONTRIBUTORS.md under the CesiumGS section!

[ErixenCruz]

I added myself in #8983. Should I add myself here, too? @hpinkos

[hpinkos]

@ErixenCruz yes please! I think this PR will be ready to be merged first, and we should add your name to the contributors list when this goes in

[ErixenCruz]

@hpinkos Pushed commit with your feedback. Thanks.

[ErixenCruz]

@lilleyse Updated.

[lilleyse]

@ErixenCruz thanks! Looks good.

[nmschulte]

This is very useful, thank you!