Refactor out CSS gradient from custom-gradient-bar

[ajlende]

Description

While working on #26361 I wanted to reuse components from the gradient picker so they can share the same style and markup.

The old gradient bar worked directly with a CSS gradient string. This refactoring introduces an intermediate color-stop array to be used in the CustomGradientBar instead of that string. Then, at the CustomGradientPicker level, the color-stop array can finally be converted back to the gradient string. This means that the gradientAST doesn't need to be passed all the way down too. The logic remains fairly similar otherwise—I didn't want to make this a complete rewrite.

How has this been tested?

• Add control points (and update the color at the same time)
• Remove control points
• Update control points
• Select gradient presets on web and mobile

Screenshots

No visual changes made. Markup generated should be identical.

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[mkaz]

I got an error when trying out this PR, when clicking a preset gradient.
See attached screencast:

[ajlende]

@mkaz Thanks for the catch! The theme I was using only provided colors as rgb so I didn't see it. All cases should be covered now.

[mkaz]

The updated changes tested well, thanks for the quick fix.
Looks good 👍