Support `transparent` Color

[Collin Heist]

Note: See pydantic/pydantic#6026 for the original issue

As of CSS3, the transparent color is supposed to be supported in all color fields, and is equivalent to rgba(0, 0, 0, 0). From the CSS3 website:

CSS3 extends the color value to include the ‘transparent’ keyword to allow its use with all properties that accept a value. This simplifies the definition of those properties in CSS3.
transparent
Fully transparent. This keyword can be considered a shorthand for transparent black, rgba(0,0,0,0), which is its computed value.

I'll submit a PR here shortly, as the implementation should be fairly simple.

[Samuel Colvin]

👍 sounds good.

[Yasser Tahiri]

Merged #59

Thank you @CollinHeist 🙏🏻