Make the core color palette opt-in for themes with not theme.json

[oandregal]

Follow-up to #36492 #35970

In #35970 we introduced the ability to show three different palettes (default, theme, user) in the color UI component. We also have a couple of flags for themes to disable this should they want (corePalette, coreGradients). However, by introducing this change, themes that don't use theme.json will now see the default colors as well.

This PR introduces the following changes:

• Hides the default palette for themes with no theme.json that declare its own palette.
• Adds new theme supports for themes with no theme.json to opt-in into the default solid and gradient palettes, so they can show both the default and the theme one:
  • default-color-palette which mimic the existing editor-color-palette theme supports
  • default-gradient-presets which mimics the existing editor-gradient-presets theme support

How to test

Test a theme that does not provide a palette:

• Use Storefront.
• Load the editor and verify that the color component shows the default palette.

Test a theme that provides a palette:

• Use TwentyTwentyOne.
• Load the editor and verify that the color component only shows the theme solids&gradients but not the default ones (they'll be shown without this PR).

Test a theme that provides a palette and opts-in into the core palette:

• Use TwentyTwentyOne.
• Add add_theme_support( 'default-color-palette' ) and add_theme_support( 'default-gradient-presets' ) in its functions.php.
• Load the editor and verify that the color component shows both the default and the theme palettes for solids & gradients.

[fabiankaegy]

Closes #36489

[Mamaduka]

It might not be accurate in the future, but it works for now.

[Mamaduka]

It works as expected.

Before	After

[oandregal]

This is not ready to land. We need to also cover the case in which the theme does not provide any theme palette: in that case, we still need to show the core one. Updated PR description with instructions for testing.

[Mamaduka]

@oandregal, I'm going to add the "In Progress", so we don't merge it by accident 😄

[oandregal]

@fabiankaegy @Mamaduka This is now ready to test.

I've run this by some folks (Matías and others) and they think it's best to add new theme supports that classic themes can use to opt-in into this, so I've done that as well. Another point of feedback was that we should rename core into defaults everywhere. I'll create a separate follow-up PR for that change.

[oandregal]

I'm going to go ahead and merge this one to unblock #36622 Please, feel free to do post-merge reviews and I'll address any issue left.

[Mamaduka]

I did another test run and works as expected 👍

[steveangstrom]

This is not fixed in Gutenberg 12.0.1 . Still broken there. I have a wall of ugly "core".

I assume that's intentional?

Is there a timeline for this patch to make it into the released plugin?

[oandregal]

Hey Steve, this is going to be part of the 12.1 release, which should be a few days away.