Adds support for color picker in css`` blocks

[au5ton]

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This adds support for CSS colors in the following formats:

• rgb(...)
• rgba(...)
• hsl(...)
• hsla(...)

I also added some rudimentary tests for the color provider as a result of these changes.

Fixes #319

[au5ton]

@runem What do you think? Is there anything I can do to improve this before it can be accepted?