Fixed color creation from CSS color string with modern "space-separated" syntax

[aerialist7]

Support Color.fromCssColorString parsing rgb(a)/hsl(a) CSS colors with modern "space-separated" syntax

Closes #11264

[cesium-concierge]

Thank you so much for the pull request @aerialist7! 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.
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• ❌ Missing CLA.
  • Please send in a Contributor License Agreement (CLA) and comment back here to let us know to check this!
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
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[aerialist7]

UPD:
✅ CONTRIBUTORS.md entry was updated
✅ CLA was confirmed
✅ CHANGES.md was not updated because there were no API changes
✅ Unit tests were updated

[aerialist7]

@ggetz, as I understand tests fail because of Node.js version

[ggetz]

Thanks for the PR @aerialist7!

I am seeing some test failures in the CI logs - https://app.travis-ci.com/github/CesiumGS/cesium/jobs/601666466.

I think there is an extra character in the regex string.

[ggetz]

@aerialist7 Once #11271 (comment) is resolved, could you please update CHANGES.md with a summary of your changes?

[aerialist7]

@aerialist7 Once #11271 (comment) is resolved, could you please update CHANGES.md with a summary of your changes?

@ggetz, done.
Also, I moved my changes to a separate branch, so there are no conflicts anymore, you can easily merge it

[ggetz]

Awesome, thanks again @aerialist7!