Add RGBA to Customizer color picker

[melchoyce]

It would be helpful to provide a UI for changing color opacity in the Customizer's color picker.

Some prior discussion about supporting RGBA in Iris here: ​https://github.com/Automattic/Iris/issues/13

See #21059

[melchoyce]

Based on reviewing the contributions to that GitHub issue, this seems like it might be the best option:

• ​https://github.com/BraadMartin/components/tree/master/customizer/alpha-color-picker
• ​http://braadmartin.com/alpha-color-picker-control-for-the-wordpress-customizer/

Can someone do a code and accessibility review?

[westonruter]

The code from @braad I think is a good polyfill extension to add alpha support, however I think for the core merge we really should be extending the core color picker in the same way that was done for adding hue slider support in #38263 and made a part of Iris itself.

@braad would you be interested in forking Automattic/Iris to extend it to add hue support, open a PR, and then create a core patch to incorporate your work into the wpColorPicker and the customizer Color control that uses it as was done for hue? This way, even if @mattwieve doesn't have time to merge it upstream, at least it would still be part of Iris although a fork.

[jbpaul17]

Punting to 4.8.1 per bug scrub earlier this week.

[westonruter]

This is not getting any traction, so punting.

[mariovalney]

#38093 was marked as a duplicate.