Add support for named orientations and literal colors on custom gradients

[jorgefilipecosta]

Fixes: #21831

This PR makes sure the custom gradient picker is able to parse and correctly display gradients with named orientations instead of an explicit angle (e.g. to right) and gradients with explicit colors (e.g: red).

How has this been tested?

I added this preset gradient:

add_theme_support( 
  'editor-gradient-presets', 
  array(
    array(
      'name'     => __( 'My gradient', 'en'  ),
      'gradient' => 'linear-gradient(to bottom, red 50%, white 100%)',
      'slug'     => 'my-gradient'
    )
  )
);

I selected the preset as the background of a button and I verified I could customize the gradient on the custom gradient picker.

[github-actions]

Size Change: +131 B (0%)

Total Size: 824 kB

Filename	Size	Change
build/components/index.js	180 kB	+131 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.02 kB	0 B
build/annotations/index.js	3.62 kB	0 B
build/api-fetch/index.js	4.08 kB	0 B
build/autop/index.js	2.82 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.61 kB	0 B
build/block-directory/style-rtl.css	760 B	0 B
build/block-directory/style.css	761 B	0 B
build/block-editor/index.js	102 kB	0 B
build/block-editor/style-rtl.css	10.3 kB	0 B
build/block-editor/style.css	10.3 kB	0 B
build/block-library/editor-rtl.css	7.12 kB	0 B
build/block-library/editor.css	7.12 kB	0 B
build/block-library/index.js	115 kB	0 B
build/block-library/style-rtl.css	7.34 kB	0 B
build/block-library/style.css	7.34 kB	0 B
build/block-library/theme-rtl.css	683 B	0 B
build/block-library/theme.css	685 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	48.1 kB	0 B
build/components/style-rtl.css	17 kB	0 B
build/components/style.css	16.9 kB	0 B
build/compose/index.js	6.66 kB	0 B
build/core-data/index.js	11.4 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.45 kB	0 B
build/date/index.js	5.47 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	3.1 kB	0 B
build/edit-navigation/index.js	4.41 kB	0 B
build/edit-navigation/style-rtl.css	618 B	0 B
build/edit-navigation/style.css	617 B	0 B
build/edit-post/index.js	28 kB	0 B
build/edit-post/style-rtl.css	12.2 kB	0 B
build/edit-post/style.css	12.2 kB	0 B
build/edit-site/index.js	12.1 kB	0 B
build/edit-site/style-rtl.css	5.22 kB	0 B
build/edit-site/style.css	5.22 kB	0 B
build/edit-widgets/index.js	8.37 kB	0 B
build/edit-widgets/style-rtl.css	4.69 kB	0 B
build/edit-widgets/style.css	4.69 kB	0 B
build/editor/editor-styles-rtl.css	425 B	0 B
build/editor/editor-styles.css	428 B	0 B
build/editor/index.js	44.3 kB	0 B
build/editor/style-rtl.css	5.07 kB	0 B
build/editor/style.css	5.08 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	734 B	0 B
build/format-library/index.js	7.63 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	2.14 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	710 B	0 B
build/keyboard-shortcuts/index.js	2.51 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.12 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	5.29 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/rich-text/index.js	14.8 kB	0 B
build/server-side-render/index.js	2.68 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.28 kB	0 B
build/url/index.js	4.02 kB	0 B
build/viewport/index.js	1.84 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.18 kB	0 B

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