Fix: theme colors cannot override defaults

[oandregal]

Fixes #36772

This PR removes the theme presets that have the same slug as a core preset of the same type. For example, the color theme presets can't have a black slug, which is already part of the color default presets.

How to test

Global presets are filtered

• Use the TT1-blocks theme.
• In its theme.json update the value of the colors whose slug is black and white to a color that is easily inspectable for debugging purposes. For example:

{
  "slug": "black",
  "color": "#FF69B4",
  "name": "Black"
},
{
  "slug": "white",
  "color": "#FF69B4",
  "name": "White"
}

• Go to the front-end and inspect the global-styles-inline-css embedded stylesheet.
  • Verify that there's only a single --wp--preset--color--black whose value is #000000 and a single --wp--preset--color--white whose value is #FFFFFF (they come from the default palette).
  • Before this PR the value for both would be #FF69B4.
• Go to the editors and verify that the theme palette does not contain the black & white colors in the theme palette.

Before	After

Block-level presets are filtered

• In the theme.json file of the TT1-blocks, add the following palette under settings.blocks.core/paragraph.color.palette:

[
  {
    "slug": "black",
    "color": "#FF69B4",
    "name": "Black"
  },
  {
    "slug": "yellow",
    "color": "#EEEADD",
    "name": "Yellow"
  },
  {
    "slug": "white",
    "color": "#FF69B4",
    "name": "White"
  }
]

• In the editors, the expected result is that the palette for the paragraph only contains one color (the yellow).
• In the front, within the global-styles-inline-css embedded stylesheet the expected result is that there's new CSS Custom Properties and classes scoped to the paragraph block:

p {
  --wp--preset--color--yellow: #eeeadd;
}

p.has-yellow-color {
  color: var(--wp--preset--color--yellow) !important;
}
p.has-yellow-background-color {
  background-color: var(--wp--preset--color--yellow) !important;
}
p.has-yellow-border-color {
  border-color: var(--wp--preset--color--yellow) !important;
}

[oandregal]

Marking as "needs dev note" to share that theme overriding theme colors is not expected and creates conflicts with the current direction for the UI color component (showing all palettes).

[jorgefilipecosta]

LGTM 👍

[oandregal]

Pushed a change to limit this to the palette and gradient presets, the only ones for which we display different sources of data at the moment.

[jorgefilipecosta]

Typo: dfefault

[jorgefilipecosta]

Did another test after the updates and things seem to work well.