fix(colors): convert rgba/hsla to rgb/hsl if opacity is >= 1

[clshortfuse]

fixes #1082

Fixes:

• rgba(R,G,B,1) => rgb(R,G,B)
• rgb(R,G,B,1) => rgb(R,G,B)
• hsla(H,S,L,1) => hsl(H,S,L)
• hsl(H,S,L,1) => hsl(H,S,L)
• Removes case-sensitivity for rgb, rgba, hsl, and hsla

For reference, CSS 4 supports alpha parameter for rgb() and hsl(). That's why the new regex is also optionally looking for a. If it finds it and opacity is 1, then it effectively removes the alpha parameter. That's a plus for backwards compatibility for browsers that don't support rgb(R G B A).

https://developer.mozilla.org/en-US/docs/Web/CSS/color_value

[jakubpawlowicz]

I wonder if this change may lead to a slight performance degradation, but let's merge it and fix if needed.

[jakubpawlowicz]

All good otherwise!

[clshortfuse]

I haven't looked at this PR in a bit, but the Regex may be incorrect and could cause issues.

The proper Regex for numbers is /[+-]?((\d+)|(\d*\.\d+))(e[+-]?\d+)?/. That covers all the requirements in the CSS spec.

I made a crazy long Regex that can verify CSS colors at https://gist.github.com/clshortfuse/acffca0a5f1412edcf9e132f33febe1c

Implementing one based along that may also fix other issues in clean-css that we aren't aware about (possibly supporting complex values like rgb( 1.2e2, 5e-4, +2).

[clshortfuse]

After closer inspection, the regex is unlikely to cause issues, but could be better. I'm making a PR to support scientific notation as well RGB(% % %). But it's not as urgent as I previously thought.