Enable alpha (opacity) in Global Styles color pickers.

[ndiego]

What?

Enables alpha (opacity) in Global Styles color pickers which fixes #42999 and #43041

Why?

Alpha was enabled previously at the block level but not in Global Styles. This PR corrects that and establishes consistency.

How?

Simply add enableAlpha to the ColorPicker component that is used in Global Styles.

Testing Instructions

1. Navigate to the Site Editor and the Global Styles panel.
2. Navigate to the Colors panel and edit a color and you will now see the alpha (opacity) slider.

Screenshots or screencast

[ndiego]

This solution was tested and proved to work as expected here.

[Mamaduka]

@ndiego, do you happen to know the PR that enabled alpha colors?

I think this change will enable alpha support everywhere the PaletteEdit component is used. Do we want this?

[ndiego]

@Mamaduka yeah it will, but it only looks like PaletteEdit is used in the Site Editor and Global Styles. Offhand, I cannot think of a reason why we wouldn't want this functionality since alpha is supported via theme.json. 🤔

I believe this is the original PR for alpha support: #37731

I did just test a few additional things, such as setting colors with opacity at the block level in Global Styles and then exporting the changes. The exported theme.json file correctly includes the alpha hex codes. Reimporting the theme and then making additional changes in Global Styles all work as expected.

I believe we should move forward with this for 13.9 and then continue testing. If we need to restrict alpha functionality elsewhere, we can prior to 6.1, but this PR moves us closer to consistency between the Editors.

[Mamaduka]

@ndiego, okay, let's give this a try. It should be easy to revert.

[ndiego]

Thanks @Mamaduka and @carolinan! 🚢ing

[priethor]

Cherry-picked to the 13.9 branch and released in Gutenberg 13.9-rc2 ✅