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.